Gas turbine operation

ABSTRACT

A aircraft gas turbine engine and operation method, the engine including: a staged combustion system having pilot and main fuel injectors, and operates in a pilot-only range wherein fuel delivers to pilot fuel injectors, and a pilot-and-main operation range wherein fuel is delivered to at least the main fuel injectors. The engine further includes a fuel delivery regulator to pilot and main fuel injectors, which receives fuel from a first and second source containing fuels each with different characteristics. The staged combustion system switches between pilot-only and pilot-and-main range operation when in steady cruise mode, the mode defining a boundary between first and second engine cruise operation range. The fuel delivery regulator delivers fuel to pilot fuel injectors during at least part of the first engine cruise operation with different fuel characteristics from fuel delivered to one or both pilot and main fuel injectors the second engine cruise operation range.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 17/853,365 filedJun. 29, 2022, which claims priority from United Kingdom PatentApplication Number 2205355.7, filed Apr. 12, 2022. The contents of eachof these applications are incorporated herein by reference in theirentirety.

BACKGROUND Field of the Disclosure

The present disclosure relates to a gas turbine engine for an aircraft,and a method of operating a gas turbine engine for an aircraft. Thepresent disclosure further relates to a computer implemented method ofdetermining one or more fuel loading parameters for an aircraft, a fuelloading parameter determination system, a method of determining afleetwide fuel allocation for a plurality of missions, and a fleetwidefuel allocation determination system. The present disclosure furtherrelates to a method of loading fuel onto the aircraft, and onto aplurality of aircraft carrying out the plurality of missions.

Description of the Related Art

There is an expectation in the aviation industry of a trend towards theuse of fuels different from the traditional kerosene-based jet fuelsgenerally used at present.

SUMMARY

According to a first aspect, there is provided a gas turbine engine foran aircraft, comprising:

-   -   a staged combustion system having pilot fuel injectors and main        fuel injectors, the staged combustion system being operable in a        pilot-only range of operation and a pilot-and-main range of        operation; and    -   a fuel delivery regulator arranged to control delivery of fuel        to the pilot and main fuel injectors, the fuel delivery        regulator being arranged to receive fuel from a first fuel        source containing a first fuel having a first fuel        characteristic and a second fuel source containing a second fuel        having a second fuel characteristic, the second fuel        characteristic being different from the first,    -   wherein the fuel delivery regulator is arranged to deliver fuel        to the pilot fuel injectors during at least part of the        pilot-only range of operation having a different fuel        characteristic from fuel delivered to one or both of the pilot        and main fuel injectors during at least part of the        pilot-and-main range of operation.

The inventors have determined that it may be advantageous to providefuel from different fuel sources having different characteristics to thefuel delivery regulator, and delivering fuel to the pilot and main fuelinjectors such that fuel of a different characteristic is provided indifferent ranges of the combustor operation. This more flexible supplyof fuel to the combustor may allow fuel which has favourable combustionproperties but is limited in supply to be used where those favourableproperties will have a greater effect.

The first fuel characteristic may be associated with a level of nvPMproduction which is less than that of the second fuel characteristic.The fuel delivered to the pilot fuel injectors during at least part ofthe pilot-only range of operation may be associated with a level of nvPMproduction which is less than that of the fuel delivered to one or bothof the pilot and main fuel injectors during at least part of thepilot-and-main range of operation.

The fuel delivery regulator may be arranged to deliver fuel from thefirst fuel source to the pilot fuel injectors during operation in boththe pilot-only and the pilot-and-main ranges of operation, and fuel fromthe second fuel source to the main fuel injectors during operation inthe pilot-and-main range of operation.

The fuel delivery regulator may be arranged to deliver fuel from thefirst fuel source to the pilot fuel injectors during operation in atleast part of the pilot-only range of operation, and deliver fuel fromthe second fuel source to the pilot fuel injectors during thepilot-and-main range of operation.

The fuel delivery regulator may be arranged to switch delivery of fuelto the pilot fuel injectors between fuel from the first fuel source andfuel from the second fuel source at one or more operating points within,or at a boundary of, the pilot-only range of operation.

The fuel delivery regulator may be arranged to switch delivery of thefuel to the pilot fuel injectors between fuel from the first fuel sourceand fuel from the second fuel source according to a mode signalindicative of a change in the range of operation of the stagedcombustion system. The switching may occur at the staging point betweenthe pilot-only range of operation and the pilot-and-main range ofoperation.

The fuel delivery regulator may be arranged to further switch deliveryof fuel to the pilot fuel injectors between fuel from the first fuelsource and fuel from the second fuel source at a threshold point withinthe pilot-only range of operation. Fuel from the second source may bedelivered to the pilot fuel injectors at fuel flow rates below thethreshold point, and fuel from the first source is delivered to thepilot fuel injectors at fuel flow rates between the threshold and theboundary of the pilot-only range of operation.

The threshold point may be a threshold fuel flow rate at which theproduction of nvPM by the gas turbine engine passes a threshold amountof the nvPM produced by the gas turbine engine during operation in whichthe pilot fuel injectors are delivered fuel having the second fuelcharacteristic

Additionally or alternatively, the threshold point may be a predefinedthreshold fuel flow rate which is less than the fuel flow rate at theboundary between pilot-only operation and pilot-and-main operation by apredefined amount. The predefined threshold fuel flow rate may be eithera percentage of the fuel flow rate at the staging point or an absolutevalue of fuel flow rate less than that at the staging point.

The fuel delivery regulator may comprise a fuel blender arranged toreceive a supply of fuel from both the first and second fuel sources andoutput fuel from the first fuel source, fuel from the second fuelsource, or a blend thereof. The fuel blender may be arranged to deliverfuel to the pilot fuel injectors.

The fuel blender may be arranged to deliver a blend of fuel from thefirst fuel source and fuel from the second fuel source to the pilot fuelinjectors during at least part of the pilot-only range of operation.

The fuel blender may be arranged to deliver a blend of fuel to the pilotfuel injectors having a varying blend ratio of fuel from the first fuelsource and fuel from the second fuel source, wherein the blend ratio maybe varied within the pilot-only range of operation according to the fuelflow rate.

The proportion of fuel from the first fuel source compared to that fromthe second fuel source may be reduced with decreasing fuel flow ratewithin the pilot-only range of operation.

The dependence on fuel flow rate of the proportion of fuel from thefirst fuel source compared to that from the second fuel source isdetermined according to a desired resultant level of nvPM at aparticular fuel flow rate, and may be determined such that the nvPM doesnot exceed a predetermined threshold or such that the nvPM production isminimised over a period of operation of the gas turbine engine. The nvPMproduction may be minimised over the LTO cycle.

The fuel blender may be arranged to blend fuel in order to achieve adesired level of nvPM at one or more operating conditions of the gasturbine engine within the pilot-only range of operation.

The fuel regulator may be arranged to deliver fuel to the pilot fuelinjectors in order to minimise a cost function dependent on one or morenvPM impact parameters. The one or more nvPM impact parameters mayinclude any one or more of:

-   -   i) height above ground level at which the nvPM production takes        place;    -   ii) location (e.g. longitude and latitude) of the nvPM        production;    -   iii) weather and/or atmospheric conditions at a location of the        nvPM production;    -   iv) climate impacts associated with location of the nvPM        production;    -   v) mass and/or size of individual nvPM particles produced;    -   vi) potential contrail production and/or contrail        characteristics;    -   vii) local air quality (LAQ) impact of nvPM production; and/or    -   viii) nvPM mass and/or number.

The blend ratio provided by the fuel blender may be determined at leastpartly according to any one or more of:

-   -   a) the amount of fuel having the first fuel composition        available for a proposed flight (e.g. estimate or measurement of        the amount of fuel in the fuel tanks making up first and second        fuel sources);    -   b) the amount of total fuel requirement for the pilot fuel        injectors during pilot-only operation for the entire flight in a        range of operation in which fuel is provided from the first fuel        source; and/or    -   c) a fuel composition limit parameter (e.g. certification limit,        fuel composition available for fuelling, aircraft/engine        limits).

According to a second aspect, there is provided a method of operating agas turbine engine for an aircraft, the gas turbine engine comprising astaged combustion system having pilot fuel injectors and main fuelinjectors, the staged combustion system being operable in a pilot-onlyrange of operation and a pilot-and-main range of operation, the methodcomprising:

-   -   regulating fuel delivery to the pilot and main fuel injectors        from a first fuel source containing a first fuel having a first        fuel characteristic and a second fuel source containing a second        fuel having a second fuel characteristic, the second fuel        characteristic being different from the first,    -   wherein the regulating of the fuel delivery comprises delivering        fuel to the pilot fuel injectors during at least part of the        pilot-only range of operation having a different fuel        characteristic from fuel delivered to one or both of the pilot        and main fuel injectors during at least part of the        pilot-and-main range of operation.

The first fuel characteristic may be associated with a level of nvPMproduction which is less than that of the second fuel characteristic.The fuel delivered to the pilot fuel injectors during at least part ofthe pilot-only range of operation may be associated with a level of nvPMproduction which is less than that of the fuel delivered to one or bothof the pilot and main fuel injectors during at least part of thepilot-and-main range of operation.

The regulating of the fuel delivery may comprise delivering fuel fromthe first fuel source to the pilot fuel injectors during operation inboth the pilot-only and the pilot-and-main ranges of operation, and fuelfrom the second fuel source to the main fuel injectors during operationin the pilot-and-main range of operation.

The regulating of the fuel delivery may comprise delivering fuel fromthe first fuel source to the pilot fuel injectors during operation in atleast part of the pilot-only range of operation, and delivering fuelfrom the second fuel source to the pilot fuel injectors during thepilot-and-main range of operation.

The regulating of the fuel delivery may comprise switching delivery ofthe fuel to the pilot fuel injectors between fuel from the first fuelsource and fuel from the second fuel source at one or more operatingpoints within, or at a boundary of, the pilot-only range of operation.

The regulating of the fuel delivery may comprise switching delivery ofthe fuel to the pilot fuel injectors between fuel from the first sourceand fuel from the second source according to a mode signal indicative ofa change in range of operation of the staged combustion system.

The regulating of the fuel delivery may further comprise switchingdelivery of fuel to the pilot fuel injectors between fuel from the firstfuel source and fuel from the second fuel source at a threshold pointwithin the pilot-only range of operation.

The threshold point may be a threshold fuel flow rate at which theproduction of nvPM by the gas turbine engine passes a threshold amountof the nvPM produced by the gas turbine engine during operation in whichthe pilot fuel injectors are delivered fuel having the second fuelcharacteristic.

Additionally or alternatively, the threshold point may be a predefinedthreshold fuel flow rate which is less than the fuel flow rate at theboundary between pilot-only operation and pilot-and-main operation by apredefined amount.

The regulating of the fuel delivery may comprise:

-   -   blending a supply of fuel from both the first and second fuel        sources to form a blended fuel consisting of fuel from the first        fuel source, fuel from the second fuel source, or a blend        thereof; and    -   delivering the blended fuel to the pilot fuel injectors.

The delivering of the blended fuel may comprise delivering a blend offuel from the first fuel source and fuel from the second fuel source tothe pilot fuel injectors during at least part of the pilot-only range ofoperation.

The delivering of the blended fuel may comprise delivering a blend offuel to the pilot fuel injectors having a varying blend ratio of fuelfrom the first fuel source and fuel from the second fuel source, whereinthe blend ratio is varied within the pilot-only range of operationaccording to the fuel flow rate.

The proportion of fuel from the first fuel source compared to that fromthe second fuel source may be reduced with decreasing fuel flow ratewithin the pilot-only range of operation.

The dependence on fuel flow rate of the proportion of fuel from thefirst fuel source compared to that from the second fuel source may bedetermined according to a desired resultant level of nvPM at aparticular fuel flow rate, and may be determined such that the nvPM doesnot exceed a predetermined threshold or such that the nvPM production isminimised over a period of operation of the gas turbine engine.

The blending of the fuel may comprise blending fuel in order to achievea desired level of nvPM at one or more operating conditions of the gasturbine engine within the pilot-only range of operation.

The regulating of the fuel delivery may comprise delivering fuel to thepilot fuel injectors in order to minimise a cost function dependent onone or more nvPM impact parameters. The one or more nvPM impactparameters may include any one or more of:

-   -   i) height above ground level at which the nvPM production takes        place;    -   ii) location of the nvPM production;    -   iii) weather and/or atmospheric conditions at the location of        the nvPM production;    -   iv) climate impacts associated with the location of the nvPM        production;    -   v) mass and/or size of individual nvPM particles produced;    -   vi) potential contrail production and/or contrail        characteristics;    -   vii) local air quality (LAQ) impact of nvPM production; and/or    -   viii) nvPM mass and/or number.

The blending of the fuel may comprise blending the fuel at a blend ratiodetermined at least partly according to any one or more of:

-   -   a) the amount of fuel having the first fuel composition        available for a proposed flight;    -   b) the amount of total fuel requirement for the pilot fuel        injectors during pilot-only operation for the entire flight in a        range of operation in which fuel is provided from the first fuel        source; and/or    -   c) a fuel composition limit parameter.

According to another aspect, there is provided an aircraft comprisingone or more gas turbine engines according to the first aspect, andoptionally any one or more of the associated statements above.

According to a third aspect, there is provided a gas turbine engine foran aircraft, comprising:

-   -   a staged combustion system having pilot fuel injectors and main        fuel injectors, the staged combustion system being operable in a        pilot-only range of operation in which fuel is delivered only to        the pilot fuel injectors, and a pilot-and-main range of        operation in which fuel is delivered to at least the main fuel        injectors; and    -   a fuel delivery regulator arranged to control fuel delivery to        the pilot and main fuel injectors, the fuel delivery regulator        being arranged to receive fuel from a first fuel source        containing a first fuel having a first fuel characteristic and a        second fuel source containing a second fuel having a second fuel        characteristic, the second fuel characteristic being different        from the first, wherein:    -   the staged combustion system is arranged to switch between the        pilot-only range of operation and the pilot-and-main range of        operation at a staging point which corresponds to a steady state        cruise mode of operation of the engine, the staging point        defining a boundary between a first engine cruise operation        range and a second engine cruise operation range; and    -   the fuel delivery regulator is arranged to deliver fuel to the        pilot fuel injectors during at least part of the first engine        cruise operation range having a different fuel characteristic        from fuel delivered to one or both of the pilot and main fuel        injectors during the second engine cruise operation range.

The inventors have determined that it may be advantageous to operate astaged combustion system so that it is in pilot-only mode during atleast some of its cruise operation, while also selectively providingfuel from two different sources to the combustor during the cruiseoperation. The inventors have determined that by setting the stagingpoint so that lower power cruise operation may take place in pilot-onlymode certain engine emissions may be reduced and combustion efficiencyimproved.

When combined with selectively using fuels having differentcharacteristics, the inventors have determined that disadvantageouseffects on emissions that would otherwise result in moving the stagingpoint can be mitigated. This therefore provides an overall improvementin combustion efficiency and reduced emissions by a combination of thesefactors.

The first fuel characteristic may be associated with a level of nvPMproduction which is less than that of the second fuel characteristic.The fuel delivered to the pilot fuel injectors during the first enginecruise operation range may be associated with a level of nvPM productionwhich is less than that of the fuel delivered to one or both of thepilot and main fuel injectors during at least part of the second enginecruise operation range.

The first fuel characteristic may correspond to a greater proportion ofSAF within the respective fuel compared to the second fuelcharacteristic, and the fuel delivered during the first engine cruiseoperation range may have a higher proportion of SAF compared to the fueldelivered during the second engine cruise operation range.

The first engine cruise operation range may correspond to operation ofthe aircraft in a later part of a cruise segment of a flight, and thesecond engine cruise operation range may correspond to operation of theaircraft in a relatively earlier part of the cruise segment.

The first engine cruise operation range may correspond to steady statesubsonic cruise operation of the engine and the second engine cruiseoperation range may correspond to steady state supersonic cruiseoperation of the engine.

The fuel delivery regulator may comprise a fuel blender arranged toreceive a supply of fuel from both the first and second fuel sources andoutput fuel from the first fuel source, fuel from the second fuelsource, or a blend thereof. The fuel blender may be arranged to deliverfuel to the pilot fuel injectors, and optionally to the main fuelinjectors.

The proportion of fuel delivered from the first fuel source compared tothat from the second fuel source may be determined according to adesired resultant level of nvPM production at a particular fuel flowrate within the first engine cruise operation range, and may bedetermined such that the nvPM production does not exceed a predeterminedthreshold or such that the nvPM production is minimised over a period ofoperation of the gas turbine engine.

The proportion of fuel delivered from the first fuel source compared tothat from the second fuel source during the first engine cruiseoperation range may be determined at least partly according to any oneor more of:

-   -   a) the amount of fuel having the first fuel characteristic and        the second fuel characteristic available for a proposed flight;    -   b) the amount of total fuel requirement for the pilot fuel        injectors during pilot-only operation for the entire flight in a        range of operation in which fuel is provided from the first fuel        source; and/or    -   c) a fuel composition limit parameter.

According to a fourth aspect, there is provided a method of operating agas turbine engine for an aircraft, the gas turbine engine comprising astaged combustion system having pilot fuel injectors and main fuelinjectors, the staged combustion system being operable in a pilot-onlyrange of operation in which fuel is delivered only to the pilot fuelinjectors, and a pilot-and-main range of operation in which fuel isdelivered to at least the main fuel injectors, the method comprising:

-   -   regulating fuel delivery to the pilot and main fuel injectors        from a first fuel source containing a first fuel having a first        fuel characteristic and a second fuel source containing a second        fuel having a second fuel characteristic, the second fuel        characteristic being different from the first;    -   switching between the pilot-only range of operation and the        pilot-and-main range of operation at a staging point during a        steady state cruise mode of operation of the engine to define a        first engine cruise operation range and a second engine cruise        operation range; and    -   delivering fuel to the pilot fuel injectors during at least part        of the first engine cruise operation range having a different        fuel characteristic from fuel delivered to one or both of the        pilot and main fuel injectors during the second engine cruise        operation range.

The first fuel characteristic may be associated with a level of nvPMproduction which is less than that of the second fuel characteristic,The fuel delivered to the pilot fuel injectors during the first enginecruise operation range may be associated with a level of nvPM productionwhich is less than that of the fuel delivered to one or both of thepilot and main fuel injectors during at least part of the second enginecruise operation range.

The first fuel characteristic may correspond to a greater proportion ofSAF within the respective fuel compared to the second fuelcharacteristic. The fuel delivered during the first engine cruiseoperation range may have a higher proportion of SAF compared to the fueldelivered during the second engine cruise operation range.

The first engine cruise operation range may correspond to operation ofthe aircraft in a later part of a cruise segment of a flight, and thesecond engine cruise operation range may correspond to operation of theaircraft in a relatively earlier part of the cruise segment.

The first engine cruise operation range may correspond to steady statesubsonic cruise operation of the engine and the second engine cruiseoperation range may correspond to steady state supersonic cruiseoperation of the engine.

Regulating fuel delivery may comprise delivering fuel from the firstfuel source, fuel from the second fuel source, or a blend thereof usinga fuel blender. The fuel blender may be arranged to deliver fuel to thepilot fuel injectors, and optionally to the main fuel injectors.

The proportion of fuel delivered from the first fuel source compared tothat from the second fuel source may be determined according to adesired resultant level of nvPM at a particular fuel flow rate withinthe first engine cruise operation range, and may be determined such thatthe nvPM does not exceed a predetermined threshold or such that the nvPMproduction is minimised over a period of operation of the gas turbineengine.

The proportion of fuel delivered from the first fuel source compared tothat from the second fuel source during the first engine cruiseoperation range may be determined at least partly according to any oneor more of:

-   -   a) the amount of fuel having the first fuel characteristic and        the second fuel characteristic available for a proposed flight;    -   b) the amount of total fuel requirement for the pilot fuel        injectors during pilot-only operation for the entire flight in a        range of operation in which fuel is provided from the first fuel        source; and/or    -   c) a fuel composition limit parameter.

According to another aspect, there is provided an aircraft comprisingone or more gas turbine engines according to the third aspect, andoptionally any one or more of the associated statements above.

According to a fifth aspect, there is provided a gas turbine engine foran aircraft, comprising:

-   -   a staged combustion system having pilot fuel injectors and main        fuel injectors, the staged combustion system being operable in a        pilot-only range of operation in which fuel is delivered only to        the pilot fuel injectors, and a pilot-and-main range of        operation in which fuel is delivered to at least the main fuel        injectors at a relative rate defined by a pilot-and-main staging        ratio; and    -   a fuel delivery regulator arranged to control delivery of fuel        to the pilot and main fuel injectors, the fuel delivery        regulator being arranged to receive fuel from a first fuel        source containing a first fuel having a first fuel        characteristic and a second fuel source containing a second fuel        having a second fuel characteristic, the second fuel        characteristic being different from the first, wherein:    -   the staged combustion system is further operable in a transition        range of operation between the pilot-only and the pilot-and-main        ranges of operation;    -   within the transition range of operation fuel is delivered to        both the pilot and main fuel injectors at a transition staging        ratio which is different from the pilot-and-main staging ratio;        and    -   the fuel delivery regulator is arranged to deliver fuel to one        or both of the pilot and main fuel injectors during the        transition range of operation having a different fuel        characteristic from fuel delivered to one or both of the pilot        and main fuel injectors during at least part of the        pilot-and-main range of operation.

The inventors have determined that a transition region can be includedthat provides a transition from the pilot-only staging ratio (100:0) tothe chosen pilot-and-main staging ratio. The inventors have determinedthat by configuring the staged combustion system to operate in atransition range between the pilot-only and pilot-and-main operation theamount of CO and HC emission operating within that range of engine powersettings can be reduced. The inventors have also determined that anydetrimental change to the engine emissions resulting from the transitionregion can be mitigated at least partly by using fuel having a differentfuel characteristic in the transition range of operation compared tothat used in at least pilot-and-main operation.

The first fuel characteristic may be associated with a level of nvPMproduction which is less than that of the second fuel characteristic.The fuel delivered to at least the pilot fuel injectors during thetransition range of operation may be associated with a level of nvPMproduction which is less than the that of fuel delivered to one or bothof the pilot and main fuel injectors during at least part of thepilot-and-main range of operation.

The first fuel characteristic may correspond to a greater proportion ofSAF within the respective fuel compared to the second fuelcharacteristic. The fuel delivered during the transition range ofoperation may have a higher proportion of SAF compared to the fueldelivered during at least part of the pilot-and-main range of operation.

The transition staging ratio may have a continuous variation withchanging engine power within at least part of the transition range ofoperation.

The continuous variation may be such that, within the transition rangeof operation, the proportion of the total fuel flow to the fuelinjectors that is attributable to fuel flow to the pilot fuel injectorsdecreases with increasing engine power and the proportion of the totalfuel flow to the fuel injectors that is attributable to fuel flow to themain fuel injectors increases with increasing engine power.

The transition staging ratio may have a constant intermediate valuewithin at least part of the transition range of operation which isdifferent from the pilot-and-main staging ratio. The constantintermediate value may be between that of the pilot-only range and thatof the pilot-and-main range.

The transition staging ratio may vary between a series of constantintermediate values (i.e. different from each other), each beingdifferent from the pilot-and-main staging ratio. Each intermediate valuemay be between that of the pilot-only range and that of thepilot-and-main range.

The fuel delivery regulator may comprise a fuel blender arranged toreceive a supply of fuel from both the first and second fuel sources andoutput fuel from the first fuel source, fuel from the second fuelsource, or a blend thereof. The fuel blender may be arranged to deliverfuel to the pilot fuel injectors, and optionally to the main fuelinjectors.

The proportion of fuel delivered from the first fuel source compared tothat from the second fuel source may be determined according to adesired resultant level of nvPM production at a particular fuel flowrate within the transition range of operation, and may be determinedsuch that the nvPM production does not exceed a predetermined thresholdor such that the nvPM production is minimised over a period of operationof the gas turbine engine.

The proportion of fuel delivered from the first fuel source compared tothat from the second fuel source during the transition range ofoperation may be determined at least partly according to any one or moreof:

-   -   a) the amount of fuel having the first fuel characteristic and        the second fuel characteristic available for a proposed flight;    -   b) the amount of total fuel requirement for the fuel injectors        during pilot-only operation for the entire flight in a range of        operation in which fuel is provided from the first fuel source;        and/or    -   c) a fuel composition limit parameter.

According to a sixth aspect, there is provided a method of operating agas turbine engine for an aircraft, the gas turbine engine comprising astaged combustion system having pilot fuel injectors and main fuelinjectors, the staged combustion system being operable in a pilot-onlyrange of operation in which fuel is delivered only to the pilot fuelinjectors, and a pilot-and-main range of operation in which fuel isdelivered to at least the main fuel injectors at a relative rate definedby a pilot-and-main staging ratio, the method comprising:

-   -   regulating fuel delivery to the pilot and main fuel injectors        from a first fuel source containing a first fuel having a first        fuel characteristic and a second fuel source containing a second        fuel having a second fuel characteristic, the second fuel        characteristic being different from the first,    -   operating the staged combustion system in a transition range of        operation between the pilot-only and the pilot-and-main ranges        of operation within which fuel is delivered to both the pilot        and main fuel injectors at a transition staging ratio which is        different from the pilot-and-main staging ratio,    -   wherein the regulating of the fuel delivery comprises delivering        fuel to one or both of the pilot and main fuel injectors during        the transition range of operation having a different fuel        characteristic from fuel delivered to one or both of the pilot        and main fuel injectors during at least part of the        pilot-and-main range of operation.

The first fuel characteristic may be associated with a level of nvPMproduction which is less than that of the second fuel characteristic.The fuel delivered to at least the pilot fuel injectors during thetransition range of operation may be associated with a level of nvPMproduction which is less than that of the fuel delivered to one or bothof the pilot and main fuel injectors during at least part of thepilot-and-main range of operation.

The first fuel characteristic may correspond to a greater proportion ofSAF within the respective fuel compared to the second fuelcharacteristic, and the fuel delivered during the transition range ofoperation may have a higher proportion of SAF compared to the fueldelivered during at least part of the pilot-and-main range of operation.

The transition staging ratio may have a continuous variation withchanging engine power within at least part of the transition range ofoperation.

The continuous variation may be such that, within the transition rangeof operation, the proportion of the total fuel flow to the fuelinjectors that is attributable to fuel flow to the pilot fuel injectorsdecreases with increasing engine power and the proportion of the totalfuel flow to the fuel injectors that is attributable to fuel flow to themain fuel injectors increases with increasing engine power.

The transition staging ratio may have a constant intermediate valuewithin at least part of the transition range of operation which isdifferent from the pilot-and-main staging ratio, and optionally liesbetween that of the pilot-only range and that of the pilot-and-mainrange.

The transition staging ratio may vary between a series of constantintermediate values, each being different from the pilot-and-mainstaging ratio, and optionally each lying between that of the pilot-onlyrange and that of the pilot-and-main range.

Regulating the fuel delivery may comprise delivering fuel from the firstfuel source, fuel from the second fuel source, or a blend thereof, usinga fuel blender. The fuel blender may be arranged to deliver fuel to thepilot fuel injectors, and optionally to the main fuel injectors.

The proportion of fuel delivered from the first fuel source compared tothat from the second fuel source may be determined according to adesired resultant level of nvPM production at a particular fuel flowrate within the transition range of operation. The proportion of fuelmay be determined such that the nvPM production does not exceed apredetermined threshold or such that the nvPM production is minimisedover a period of operation of the gas turbine engine.

The proportion of fuel delivered from the first fuel source compared tothat from the second fuel source during the transition range ofoperation may be determined at least partly according to any one or moreof:

-   -   a) the amount of fuel having the first fuel characteristic and        the second fuel characteristic available for a proposed flight;    -   b) the amount of total fuel requirement for the fuel injectors        during pilot-only operation for the entire flight in a range of        operation in which fuel is provided from the first fuel source;        and/or    -   c) a fuel composition limit parameter.

According to another aspect, there is provided an aircraft comprisingone or more gas turbine engines according to the fifth aspect, andoptionally any one or more of the associated statements above.

According to a seventh aspect, there is provided a gas turbine enginefor an aircraft, comprising:

-   -   a staged combustion system having pilot fuel injectors and main        fuel injectors, the staged combustion system being operable in a        pilot-only range of operation in which fuel is delivered only to        the pilot fuel injectors, and a pilot-and-main range of        operation in which fuel is delivered to at least the main fuel        injectors; and    -   a fuel delivery regulator arranged to control the delivery of        fuel to the pilot and main fuel injectors, the fuel delivery        regulator being arranged to receive fuel from a first fuel        source containing a first fuel having a first fuel        characteristic, and a second fuel source containing a second        fuel having a second fuel characteristic, the second fuel        characteristic being different from the first, wherein:    -   the staged combustion system is arranged to operate in an        acceleration mode in which acceleration of the engine from a        steady state mode of operation is caused; and    -   the fuel delivery regulator is arranged to deliver fuel to one        or both of the pilot and main fuel injectors, during operation        in at least a part of the acceleration mode, having a different        fuel characteristic from fuel delivered to one or both of the        pilot and main fuel injectors during at least a part of the        steady state mode of operation.

To reduce the production of excessive amounts of nvPM duringacceleration, it is known to switch to an “acceleration” mode ofoperation of a gas turbine engine in which the staging point occurs at alower engine power setting. The inventors have observed that switchingto such a known acceleration mode however may have a number ofdrawbacks. For example, an increase in HC and CO emissions may becaused. In an acceleration mode of the present application the fueldelivery regulator is arranged to deliver fuel to the fuel injectors(i.e. the pilot and/or main fuel injectors) having a different fuelcharacteristic from fuel delivered to the fuel injectors (i.e. the pilotand/or main fuel injectors) during at least a part of the steady statemode of operation. The inventors have determined that increased nvPMemissions when the engine operates in the acceleration mode can beavoided or reduced by using a fuel with different characteristics fromthat which is used during steady state operation. This may allow thestaging point during the acceleration mode to remain the same or similarto that of the steady state mode of operation, thus avoiding orreducing/limiting any disadvantageous increase in HC or CO emissions.

The first fuel characteristic may be associated with a level of nvPMproduction which is less than that of the second fuel characteristic.The fuel delivered to at least the pilot fuel injectors during theacceleration mode may be associated with a level of nvPM productionwhich is less than that of the fuel delivered to one or both of thepilot and main fuel injectors during operation in the steady state mode.

The first fuel characteristic may correspond to a greater proportion ofSAF within the respective fuel compared to the second fuelcharacteristic, and the fuel delivered during the acceleration mode mayhave a higher proportion of SAF.

The staged combustion system may be arranged to switch between operationin the pilot-only and pilot-and-main ranges of operation at a stagingpoint.

The staging point may be at the same or higher engine power in theacceleration mode compared to the steady state mode.

The staging point may be at a lower engine power in the accelerationmode compared to the steady state mode and is at a higher power than adefault staging point according to which the staged combustion system iscontrolled where fuel of a different characteristics cannot be providedto the combustion system.

The fuel delivery regulator may be arranged to deliver fuel to the pilotfuel injectors during pilot-only operation in the acceleration mode thathas a different fuel characteristic from fuel delivered to the main fuelinjectors during pilot-and-main operation in the steady state mode ofoperation of the engine.

The fuel delivery regulator may be arranged to deliver fuel during thepilot-only range of operation in the acceleration mode having a fuelcharacteristic determined based on a control parameter on which nvPMproduction by the engine is dependent.

The control parameter may be a fuel-to-air ratio in a combustor of thestaged combustion system.

As the fuel-to-air ratio decreases, the proportion of fuel associatedwith low nvPM production delivered to the pilot fuel injectors may alsobe decreased.

The fuel delivery regulator may be arranged to switch delivery of fuelto one or both of the main and pilot fuel injectors to that having adifferent fuel characteristic at a start point of a period of operationin the acceleration mode.

The fuel delivery regulator may be arranged to return to delivery offuel having the same fuel characteristic as that delivered in the steadystate mode following a transition to pilot-and-main operation.

The fuel delivery regulator may be arranged to deliver fuel to one orboth of the pilot and main fuel injectors during the acceleration modeat a rate greater than that sufficient to maintain steady-stateoperation of the engine.

According to an eighth aspect, there is provided a method of operating agas turbine engine for an aircraft, the gas turbine engine comprising astaged combustion system having pilot fuel injectors and main fuelinjectors, the staged combustion system being operable in a pilot-onlyrange of operation in which fuel is delivered only to the pilot fuelinjectors, and a pilot-and-main range of operation in which fuel isdelivered to at least the main fuel injectors, the method comprising:

-   -   regulating fuel delivery to the pilot and main fuel injectors        from a first fuel source containing a first fuel having a first        fuel characteristic and a second fuel source containing a second        fuel having a second fuel characteristic, the second fuel        characteristic being different from the first;    -   operating the staged combustion system in an acceleration mode        in which acceleration of the engine from a steady state mode of        operation is caused; and    -   delivering fuel to one or both of the pilot and main fuel        injectors, during operation in at least a part of the        acceleration mode, having a different fuel characteristic from        fuel delivered to one or both of the pilot and main fuel        injectors during at least a part of the steady state mode of        operation.

The first fuel characteristic may be associated with a level of nvPMproduction which is less than that of the second fuel characteristic,and the fuel delivered to at least the pilot fuel injectors during theacceleration mode may be associated with a level of nvPM productionwhich is less than that of the fuel delivered to one or both of thepilot and the main fuel injectors during operation in the steady statemode.

The first fuel characteristic may correspond to a greater proportion ofSAF within the respective fuel compared to the second fuelcharacteristic, and the fuel delivered during the acceleration mode mayhave a higher proportion of SAF.

The staged combustion system may be arranged to switch between operationin the pilot-only and pilot-and-main ranges of operation at a stagingpoint.

The staging point may be at the same or higher engine power in theacceleration mode compared to the steady state mode.

The staging point may be at a lower engine power in the accelerationmode compared to the steady state mode, and may be at a higher powerthan a default staging point according to which the staged combustionsystem is controlled where fuel of different fuel characteristic cannotbe provided to the combustion system.

Fuel may be delivered to the pilot fuel injectors during pilot-onlyoperation in the acceleration mode that has a different fuelcharacteristic from fuel delivered to the main fuel injectors duringpilot-and-main operation in the steady state mode of operation of theengine.

The delivering of fuel to the fuel injectors may comprise deliveringfuel during the pilot-only range of operation in the acceleration modehaving fuel characteristics based on a control parameter on which nvPMproduction by the engine is dependent. The control parameter may be afuel-to-air ratio in a combustor of the staged combustion system. As thefuel-to-air ratio decreases, the proportion of fuel associated with lownvPM production delivered to the pilot fuel injectors may also bedecreased.

The delivering of fuel to the fuel injectors may comprise switching thedelivery of fuel to one or both of the main and pilot fuel injectors tothat having a different fuel characteristic at a start point of a periodof operation in the acceleration mode.

The delivering of fuel to the fuel injectors may comprise returning todelivery of fuel having the same fuel characteristic as that deliveredin the steady state mode following a transition to pilot-and-mainoperation.

During the acceleration mode fuel may be delivered to the fuel injectorsat a rate greater than that sufficient to maintain steady-stateoperation.

According to another aspect, there is provided an aircraft comprisingone or more gas turbine engines according to the seventh aspect, andoptionally any one or more of the associated statements above.

According to a ninth aspect, there is provided a gas turbine engine foran aircraft, comprising:

-   -   a staged combustion system having pilot fuel injectors and main        fuel injectors, the staged combustion system being operable in a        pilot-only range of operation and a pilot-and-main range of        operation; and    -   a fuel delivery regulator arranged to control delivery of fuel        to the pilot and main fuel injectors;    -   a fuel characteristic determination module configured to        determine one or more fuel characteristics of the fuel being        supplied to the staged combustion system; and    -   a controller configured to determine a staging point defining        the point at which the staged combustion system is switched        between pilot-only operation and pilot-and-main operation, the        staging point being determined based on the determined one or        more fuel characteristics, and the controller being configured        to control the staged combustion system according to the        determined staging point.

The inventors have determined the staging point according to which astaged combustion system is controlled can be determined according tothe characteristics of the fuel being supplied to the combustor. Asdiscussed above in connection with the third, fourth, seventh and eighthaspects, the staging point can be controlled in cases where fuel ofdifferent characteristics is available to reduce certain engineemissions.

The one or more fuel characteristics may indicate that the fuel isassociated with a lower nvPM production level compared to fossilkerosene.

The one or more fuel characteristics may include any one or more of:

-   -   (i) a percentage of sustainable aviation fuel in the fuel;    -   (ii) an aromatic hydrocarbon content of the fuel; and/or    -   (iii) a naphthalene content of the fuel.

The controller may be configured to determine the staging point suchthat a staging point associated with one or more fuel characteristicsthat indicate that the fuel is associated with a low nvPM productioncorresponds to a higher engine power setting compared to a staging pointassociated with one or more fuel characteristics that indicate that thefuel is associated with a relatively higher nvPM production.

The determined staging point may be a cruise staging point, and thecontroller may be configured to control the combustion system using thedetermined staging point during a cruise operating condition of theengine.

The controller may be configured to determine the staging point suchthat the staged combustion system is arranged to switch between thepilot-only range of operation and the pilot-and-main range of operationat a staging point which corresponds to a steady state cruise mode ofoperation of the engine, the staging point defining a boundary between afirst engine cruise operation range and a second engine cruise operationrange.

The first cruise operation range may correspond to operation of theaircraft in a later part of a cruise segment of a flight, and the secondoperation range may correspond to operation of the aircraft in arelatively earlier part of the cruise segment.

The first cruise operation range may correspond to steady state subsoniccruise operation of the engine and the second cruise operation rangecorresponds to steady state supersonic cruise operation of the engine.

The determined staging point may be an engine acceleration stagingpoint, and the controller may be configured to control the stagedcombustion system using the determined staging point during anacceleration operating condition of the engine. The engine accelerationstaging point may be determined to be the same (e.g. set to be the same)as a cruise staging point in response to the one or more fuelcharacteristics.

According to a tenth aspect, there is provided a method of operating agas turbine engine for an aircraft, the gas turbine engine comprising astaged combustion system having pilot fuel injectors and main fuelinjectors, the staged combustion system being operable in a pilot-onlyrange of operation and a pilot-and-main range of operation, the methodcomprising:

-   -   determining one or more fuel characteristics of a fuel being        supplied to the staged combustion system;    -   determining a staging point, defining the point at which the        staged combustion system is switched between pilot-only        operation and pilot-and-main operation, based on the determined        one or more fuel characteristics; and    -   controlling the staged combustion system according to the        determined staging point.

The one or more fuel characteristics may indicate that the fuel isassociated with a lower nvPM production level compared to fossilkerosene.

The one or more fuel characteristics include any one or more of:

-   -   (i) a percentage of sustainable aviation fuel in the fuel;    -   (ii) an aromatic hydrocarbon content of the fuel; and/or    -   (iii) a naphthalene content of the fuel.

Determining the staging point may comprise determining the staging pointsuch that a staging point associated with one or more fuelcharacteristics that indicate that the fuel is associated with a lownvPM production corresponds to a higher engine power setting compared toa staging point associated with one or more fuel characteristics thatindicate that the fuel is associated with a relatively higher nvPMproduction.

The determined staging point may be a cruise staging point, and thecombustion system may be controlled using the determined staging pointduring a cruise operating condition of the engine.

The staging point may be determined such that the staged combustionsystem is arranged to switch between the pilot-only range of operationand the pilot-and-main range of operation at a staging point whichcorresponds to a steady state cruise mode of operation of the engine.The staging point may define a boundary between a first engine cruiseoperation range and a second engine cruise operation range.

The first cruise operation range may correspond to operation of theaircraft in a later part of a cruise segment of a flight, and the secondcruise operation range may correspond to operation of the aircraft in arelatively earlier part of the cruise segment.

The first cruise operation range may correspond to steady state subsoniccruise operation of the engine and the second cruise operation range maycorrespond to steady state supersonic cruise operation of the engine.

The determined staging point may be an engine acceleration stagingpoint, and the staged combustion system may be controlled using thedetermined staging point during an acceleration operating condition ofthe engine. The engine acceleration staging point may be determined tobe the same as a cruise staging point in response to the one or morefuel characteristics.

According to an eleventh aspect, there is provided an aircraftcomprising one or more gas turbine engines according to the ninthaspect, and optionally any one or more of the associated statementsabove.

According to a twelfth aspect, there is provided a gas turbine enginefor an aircraft, comprising:

-   -   a staged combustion system having pilot fuel injectors and main        fuel injectors, the staged combustion system being operable in a        pilot-only range of operation and a pilot-and-main range of        operation;    -   a fuel delivery regulator arranged to control delivery of fuel        to the pilot and main fuel injectors;    -   a fuel characteristic determination module configured to        determine one or more fuel characteristics of the fuel being        supplied to the staged combustion system; and    -   a controller configured to determine a staging ratio defining        the ratio of pilot fuel injector fuel flow to main fuel injector        fuel flow, the staging ratio being determined according to the        one or more fuel characteristics, and the controller being        configured to control the staged combustion system according to        the determined staging ratio.

The inventors have determined that the staging ratio may be determinedbased on the one or more fuel characteristics in order to moreeffectively manage engine emissions. This may, for example, allow thestaging ratio to be adjusted to reduce CO and HC production in a waythat would otherwise lead to high levels of nvPM production as discussedin connection with the fifth and sixth aspect above.

The one or more fuel characteristics may indicate that the fuel isassociated with a lower nvPM production level compared to fossilkerosene.

The one or more fuel characteristics may include any one or more of:

-   -   (i) a percentage of sustainable aviation fuel in the fuel;    -   (ii) an aromatic hydrocarbon content of the fuel; and/or    -   (iii) a naphthalene content of the fuel.

The controller may be configured to:

-   -   control the staged combustion system during the pilot-and-main        range of operation according to a pilot-and-main staging ratio,        wherein the staging ratio determined according to the one or        more fuel characteristics is a transition staging ratio; and    -   control the staged combustion system so that it is operated in a        transition range of operation between the pilot-only range of        operation and the pilot-and-main range of operation, wherein        within the transition range of operation the staged combustion        system is controlled according to the transition staging ratio,        the transition staging ratio being different from the        pilot-and-main staging ratio.

The transition staging ratio may have a continuous variation withchanging engine power within the transition range of operation.

The continuous variation may be such that the proportion of the totalfuel flow to the fuel injectors that is attributable to fuel flow to thepilot fuel injectors decreases with increasing engine power. Theproportion of the total fuel flow to the fuel injectors that isattributable to fuel flow to the main fuel injectors may increase withincreasing engine power within the transition range of operation.

The transition staging ratio may vary between a series of constantintermediate values, each being different from the pilot-and-mainstaging ratio. Each intermediate value may lie between that of thepilot-only range and that of the pilot-and-main range.

According to a thirteenth aspect, there is provided a gas turbine enginecomprising a staged combustion system having pilot fuel injectors andmain fuel injectors, the staged combustion system being operable in apilot-only range of operation and a pilot-and-main range of operation,the method comprising:

-   -   determining one or more fuel characteristics of a fuel being        supplied to the staged combustion system;    -   determining a staging ratio defining the ratio of pilot fuel        injector fuel flow to main fuel injector fuel flow, the staging        ratio being determined according to the one or more fuel        characteristics; and    -   controlling the staged combustion system according to the        determined staging ratio.

The one or more fuel characteristics may indicate that the fuel isassociated with a lower nvPM production level compared to fossilkerosene.

The one or more fuel characteristics may include any one or more of:

-   -   (i) a percentage of sustainable aviation fuel in the fuel;    -   (ii) an aromatic hydrocarbon content of the fuel; and/or    -   (iii) a naphthalene content of the fuel.

Determining the staging ratio may comprise determining a transitionstaging ratio; and

-   -   controlling the staged combustion system may comprise        controlling the staged combustion system so that it is operated        in a transition range of operation between the pilot-only range        of operation and the pilot-and-main range of operation. Within        the transition range of operation the staged combustion system        may be controlled according to the transition staging ratio, the        transition staging ratio may be different from a pilot-and-main        staging ratio according to which the staged combustor is        controlled during the pilot-and-main range of operation.

The transition staging ratio may have a continuous variation withchanging engine power within the transition range of operation.

The continuous variation may be such that the proportion of the totalfuel flow to the fuel injectors that is attributable to fuel flow to thepilot fuel injectors decreases with increasing engine power, and theproportion of the total fuel flow to the fuel injectors that isattributable to fuel flow to the main fuel injectors increases withincreasing engine power within the transition range of operation.

The transition staging ratio may vary between a series of constantintermediate values, each being different from the pilot-and-mainstaging ratio. Each intermediate value may lie between that of thepilot-only range and that of the pilot-and-main range.

According to a fourteenth aspect, there is provided an aircraftcomprising one or more gas turbine engines according to the thirteenthaspect, and optionally any one or more of the associated statementsabove.

According to a fifteenth aspect, there is provided a computerimplemented method of determining a fuel allocation for an aircraft,wherein:

-   -   the aircraft comprises a first fuel source adapted to contain a        first fuel having a first fuel characteristic and a second fuel        source adapted to contain a second fuel having a second fuel        characteristic, the second fuel characteristic being different        from the first;    -   the aircraft comprises one or more gas turbine engines powered        by fuel from the first and second fuel sources;    -   the one or more gas turbine engines each comprise a staged        combustion system having pilot fuel injectors and main fuel        injectors, the staged combustion system being operable in a        pilot-only range of operation and a pilot-and-main range of        operation;    -   the one or more gas turbine engines each comprise a fuel        delivery regulator arranged to control delivery of fuel to the        pilot and main fuel injectors from the first fuel source and the        second fuel source,    -   the method comprising:    -   obtaining a proposed mission description comprising a list of        operating points for the one or more gas turbine engines during        the mission;    -   obtaining nvPM (non-volatile particulate matter) impact        parameters for the one or more gas turbine engines, the impact        parameters being associated with each operating point of the        proposed mission using fuel from the first fuel source, fuel        from the second fuel source, or a blend thereof;    -   calculating an optimised set of one or more fuel characteristics        for each operating point of the proposed mission defined in the        mission description based on the nvPM impact parameters; and    -   determining a fuel allocation based on the optimised set of one        or more fuel characteristics.

The inventors have determined that by calculating the fuel allocation inthis way, fuel can be allocated to a mission such that fuel having therequired characteristics can be provided to the aircraft for it to carryout the proposed mission while reducing the nvPM impact. This may allowbetter use of the characteristics of the fuel available in reducing nvPMcompared to loading a set amount of different types of fuel available,regardless of the mission that is to be performed using that fuel.

The first fuel characteristic may be associated with a level of nvPMproduction which is less than that of the second fuel characteristic.

Additionally or alternatively, the first fuel characteristic and thesecond fuel characteristic may be a percentage of SAF present in therespective fuel.

Each operating point of the mission description may include any one ormore of: one or more operating conditions in which the gas turbineengines are to operate, one or more fuel flow rate values correspondingto an operating point, and/or a time duration of operation at acorresponding operating point.

The nvPM impact parameters may include an nvPM impact parameter definingan amount of nvPM produced by the respective gas turbine engine fordifferent respective fuel characteristics comprising the first fuel, thesecond fuel, or a blend thereof at each operating point of the flightdescription.

The fuel allocation may include any one or more of:

-   -   i) an amount of fuel allocated to each of the first and second        fuel sources;    -   ii) the first fuel characteristic;    -   iii) the second fuel characteristic; and/or    -   iv) a fuel mixing ratio.

The method may further comprise determining one or more fuel usageparameters corresponding to the fuel allocation. The fuel usageparameters may define how the fuel is to be used during the missiondefined by the mission description. The one or more fuel usageparameters may include any one or more of:

-   -   i) a blending schedule according to which fuel from the first        fuel source and the second fuel source is blended by the fuel        delivery regulator;    -   ii) a switching schedule according to which the fuel delivery        regulator is configured to switch between delivery of fuel from        the first fuel source and the second fuel source;    -   iii) an allocation of fuel tanks provided in the aircraft to        form the first fuel source and the second fuel source; and/or    -   iv) an isolation valve setting for fuel tanks forming the first        fuel source and the second fuel source.

The optimised set of one or more fuel characteristics may be furtherdetermined based on any one or more of:

-   -   i) the achievable range of fuel characteristics that can be        provided by the fuel delivery regulator;    -   ii) a total quantity of a non-default fuel allocated to the        mission;    -   iii) a total fuel requirement for the mission;    -   iv) the capacities of the fuel tanks of the aircraft; and/or    -   v) restrictions on how the aircraft fuel tanks can be allocated        to the first or the second fuel source.

Calculating the optimised set of one or more fuel characteristics maycomprise minimising a cost function dependent on the one or more nvPMimpact parameters.

The one or more nvPM impact parameters may include any one or more of:

-   -   i) height above ground level at which the nvPM production takes        place;    -   ii) location of the nvPM production;    -   iii) weather and/or atmospheric conditions at a location of the        nvPM production;    -   iv) climate impacts associated with location of the nvPM        production;    -   v) mass/size of individual nvPM particles produced;    -   vi) potential contrail production and/or contrail        characteristics; vii) local air quality (LAQ) impact of nvPM        production; and/or    -   viii) amount of nvPM produced (e.g. mass and/or number).

According to a sixteenth aspect, there is provided a method of loadingfuel onto an aircraft, comprising:

-   -   determining a fuel allocation using the method of the fifteenth        aspect and optionally any one or more of the associated        statements above; and    -   loading fuel onto the aircraft according to the fuel allocation.

According to a seventeenth aspect, there is provided a non-transitorycomputer readable medium having stored thereon instructions that, whenexecuted by a processor, cause the processor to perform the method ofthe fifteenth aspect and optionally any one or more of the associatedstatements above.

According to an eighteenth aspect, there is provided a fuel allocationdetermination system for determining a fuel allocation for an aircraft,the fuel allocation determination system comprising a computing deviceconfigured to perform the method of the fifteenth aspect and optionallyany one or more of the associated statements above.

According to a nineteenth aspect, there is provided a fuel allocationdetermination system for determining a fuel allocation for an aircraft,wherein:

-   -   the aircraft comprises a first fuel source adapted to contain a        first fuel having a first fuel characteristic and a second fuel        source adapted to contain a second fuel having a second fuel        characteristic, the second fuel characteristic being different        from the first;    -   the aircraft comprises one or more gas turbine engines powered        by fuel from the first and second fuel sources;    -   the one or more gas turbine engines each comprise a staged        combustion system having pilot fuel injectors and main fuel        injectors, the staged combustion system being operable in a        pilot-only range of operation and a pilot-and-main range of        operation;    -   the one or more gas turbine engines each comprise a fuel        delivery regulator arranged to control delivery of fuel to the        pilot and main fuel injectors from the first fuel source and the        second fuel source,    -   the fuel loading parameter determination system comprising:    -   a mission description obtaining module configured to obtain a        proposed mission description comprising a list of operating        conditions for the one or more gas turbine engines during the        mission;    -   an impact parameter obtaining module configured to obtain nvPM        impact parameters for the one or more gas turbine engines, the        impact parameters being associated with each operating point of        the proposed mission using compositions of fuel which include        fuel from the first fuel source, fuel from the second fuel        source, or a blend thereof;    -   a fuel characteristic calculating module configured to calculate        an optimised set of one or more fuel characteristics for each        operating point of the proposed mission defined in the mission        description based on the nvPM impact parameters; and    -   a fuel allocation determining module configured to determine a        fuel allocation based on the optimised set of one or more fuel        characteristics.

The first fuel characteristic may be associated with a level of nvPMproduction which is less than that of the second fuel characteristic.

Additionally or alternatively, the first fuel characteristic and thesecond fuel characteristic may be a percentage of SAF present in therespective fuel.

Each of the operating points of the mission description obtained by themission description obtaining module may include any one or more of: oneor more operating conditions in which the gas turbine engines are tooperate, one or more fuel flow rate values corresponding to an operatingpoint, and/or a time duration of operation at a corresponding operatingpoint.

The nvPM impact parameters obtained by the impact parameter obtainingmodule may include an nvPM impact parameter defining an amount of nvPMproduced by the gas turbine engines for different respective fuelcharacteristics comprising the first fuel, the second fuel, or a blendthereof at each operating point of the mission description.

The fuel allocation determined by the fuel allocation determining modulemay include any one or more of:

-   -   i) an amount of fuel allocated to each of the first and second        fuel sources;    -   ii) the first fuel characteristic;    -   iii) the second fuel characteristic; and/or    -   iv) a fuel mixing ratio.

The fuel allocation determination system may further comprise a fuelusage parameter determining module configured to determine one or morefuel usage parameters corresponding to the fuel allocation, the fuelusage parameters defining how the fuel is to be used during the missiondefined by the mission description.

The one or more fuel usage parameters may optionally include any one ormore of:

-   -   i) a blending schedule according to which fuel from the first        fuel source and the second fuel source is blended by the fuel        delivery regulator;    -   ii) a switching schedule according to which the fuel delivery        regulator is configured to switch between delivery of fuel from        the first fuel source and the second fuel source;    -   iii) an allocation of fuel tanks provided in the aircraft to        form the first fuel source and the second fuel source; and/or    -   iv) an isolation valve setting for fuel tanks forming the first        fuel source and the second fuel source.

The optimised set of one or more fuel characteristics determined by thefuel characteristic calculating module may be further determined basedon any one or more of:

-   -   i) the achievable range of fuel characteristics that can be        provided by the fuel delivery regulator;    -   ii) a total quantity of a non-default fuel allocated to the        mission;    -   iii) a total fuel requirement for the mission;    -   iv) the capacities of the fuel tanks of the aircraft; and/or    -   v) restrictions on how the aircraft fuel tanks can be allocated        to the first or the second fuel source.

The fuel characteristic calculating module may be configured tocalculate the optimised set of one or more fuel characteristics byminimising a cost function dependent on the one or more nvPM impactparameters.

The one or more nvPM impact parameters may include any one or more of:

-   -   i) height above ground level at which the nvPM production takes        place;    -   ii) location of the nvPM production;    -   iii) weather and/or atmospheric conditions at a location of the        nvPM production;    -   iv) climate impacts associated with location of the nvPM        production;    -   v) mass of the nvPM particles produced;    -   vi) potential contrail production and/or contrail        characteristics;    -   vii) local air quality (LAQ) impact of nvPM production; and/or    -   viii) amount of nvPM produced (e.g. mass/number).

According to a twentieth aspect, there is provided an aircraftcomprising:

-   -   a first fuel source adapted to contain a first fuel having a        first fuel characteristic and a second fuel source adapted to        contain a second fuel having a second fuel characteristic, the        second fuel characteristic being different from the first;    -   one or more gas turbine engines powered by fuel from the first        and second fuel sources, wherein:        -   the one or more gas turbine engines each comprise a staged            combustion system having pilot fuel injectors and main fuel            injectors, the staged combustion system being operable in a            pilot-only range of operation and a pilot-and-main range of            operation;        -   the one or more gas turbine engines each comprise a fuel            delivery regulator arranged to control delivery of fuel to            the pilot and main fuel injectors from the first fuel            source, the second fuel source, or a blend thereof; and    -   a fuel allocation determination system according to the        eighteenth or nineteenth aspect and optionally any one or more        of the associated statements above.

According to a twenty first aspect, there is provided a computerimplemented method of determining a fleetwide fuel allocation for aplurality of missions carried out by a plurality of aircraft, theplurality of missions being supplied with fuel from a fuel sourcecomprising an amount of a default fuel and an amount of a non-defaultfuel, the fuel allocation indicating the amount of the non-default fueland the default fuel to be allocated to each of the plurality ofmissions, the default fuel and the non-default fuel having one or morefuel characteristics different from each other, the method comprising:

-   -   obtaining an initial proposed fuel allocation for each of the        plurality of missions;    -   performing a fleet-wide optimisation in which the proposed fuel        allocation of each of the plurality of missions is modified        within the constraints of the total available default and/or        non-default fuel from the fuel source to minimise a sum of        per-mission nvPM impact parameters over all of the plurality of        missions, each of the plurality of missions being associated        with a respective per-mission nvPM impact parameter determined        according to a fuel usage for that mission, the fuel usage        defining how the fuel allocation for the respective mission is        to be used during that mission; and    -   determining the fleetwide fuel allocation for the plurality of        missions based on the fleet-wide optimisation.

The inventors have determined that available fuel for a plurality ofmissions can be intelligently shared between those missions to makeadvantageous use of different types of fuel available. This may allowfuel of which there is less available to be shared between the missionsin order to use it more effectively e.g. such that the overall nvPMimpact of the missions is reduced.

The non-default fuel may be associated with a level of nvPM productionwhich is less than that of the default fuel.

The non-default fuel may be formed from a mixture of a first fuel havinga first fuel characteristic and a second fuel having a second fuelcharacteristic, different from the first.

The first and second fuel characteristics may be a percentage of SAFwithin the respective fuel. The non-default fuel may be a SAF-rich fueland the default fuel may be a relatively SAF-poor fuel (i.e. having alower SAF content compared to the SAF rich fuel).

Performing the fleet-wide optimisation may comprise:

-   -   i) performing an outer-loop optimisation in which the fuel        allocation of one or more of the plurality of missions is varied        to reduce the sum of the per-mission nvPM impact parameters of        the plurality of missions; and    -   ii) performing an inner-loop optimisation in which the fuel        usage for each of the plurality of missions is obtained        according to the constraints of the varied fuel allocation to        determine a new proposed fuel usage for each of the plurality of        missions.

Steps i) and ii) may be repeated until an optimised fuel usage for eachof the plurality of missions is determined which corresponds to aminimised sum of the per-mission nvPM impact parameters.

The inner-loop optimisation may comprise obtaining a pre-preparedsolution for the fuel usage for a respective mission.

The proposed fuel allocation for each of the plurality of missions maybe obtained by obtaining an optimised fuel usage for the respectivemission defining how the fuel is to be used to minimise the per-missionnvPM impact parameter for that mission.

The optimised fuel usage for each mission may be obtained by performinga per-mission optimisation.

The per-mission optimisation may comprise, for each respective mission:

-   -   determining a type and/or operational capabilities of a        combustor used by the respective aircraft used for the mission;    -   determining a total fuel requirement for the respective mission;    -   determining an amount of fuel required for each type of fuel        injector provided in the combustor for the respective mission        where more than one type of injector is provided;    -   determining the dependence of nvPM emissions for each engine        operating point of the mission using fuel having the        characteristics of the default fuel, non-default fuel, or a        mixture thereof; and    -   determining an optimised fuel usage which minimises the total        nvPM emissions for the respective mission.

Determining a type of combustor used by the aircraft may comprisedetermining whether the aircraft comprises a lean-burn staged combustoror a rich-burn combustor.

If the combustor is a lean-burn staged combustor having pilot and mainfuel injectors, determining an amount of fuel required for each type offuel injector may comprise:

-   -   a) determining an amount of fuel required for the pilot        injectors during pilot-and-main operation; and/or    -   b) determining an amount of fuel required for the pilot fuel        injectors during pilot-only operation; and/or    -   c) determining an amount of fuel required for the pilot fuel        injectors operating within a threshold range of the operation at        fuel flow rates below that of the staging point.

The fleet-wide optimisation may be based on:

-   -   a percentage of a first fuel having a first fuel characteristic        within the default fuel defining the lowest possible percentage        of fuel having the first fuel characteristic which can be used        for combustion; and/or    -   a percentage of the first fuel having the first fuel        characteristic within the non-default fuel defining the highest        possible percentage of fuel having the first fuel characteristic        which can be used for combustion; and/or    -   the quantity of non-default fuel available for the plurality of        missions.

According to a twenty second aspect, there is provided a method ofloading fuel onto a plurality of aircraft carrying out a plurality ofmissions, the plurality of missions being supplied with fuel from a fuelsource comprising an amount of a default fuel and an amount of anon-default fuel, the method comprising:

-   -   determining fuel allocation for the plurality of missions using        the method of the twenty first aspect and optionally any one or        more of the associated statements above; and    -   loading fuel onto the plurality of aircraft according to the        fuel allocation.

According to a twenty third aspect, there is provided a non-transitorycomputer readable medium having stored thereon instructions that, whenexecuted by a processor, cause the processor to perform the method ofthe twenty first aspect and optionally any one or more of the associatedstatements above.

According to a twenty fourth aspect, there is provided a fleetwide fuelallocation determination system for determining a fleet fuel allocationfor a plurality of missions, the fleetwide fuel allocation determinationsystem comprising a computing device configured to perform the method ofthe twenty first aspect and optionally any one or more of the associatedstatements above.

According to a twenty fifth aspect, there is provided a fleetwide fuelallocation determination system for determining a fuel allocation for aplurality of missions carried out by a plurality of aircraft, theplurality of missions being supplied with fuel from a fuel sourcecomprising an amount of a default fuel and an amount of a non-defaultfuel, the fuel allocation indicating the amount of the non-default fueland the default fuel to be allocated to each of the plurality ofmissions, the default fuel and the non-default fuel having one or morefuel characteristics different from each other, the system comprising:

-   -   an initial proposed fuel allocation obtaining module configured        to obtain an initial proposed fuel allocation for each of the        plurality of missions;    -   a fleetwide optimisation module configured to perform a        fleet-wide optimisation in which the proposed fuel allocation of        each of the plurality of missions is modified within the        constraints of the total available default and/or non-default        fuel from the fuel source to minimise a sum of per-mission nvPM        impact parameters over all of the plurality of missions, each of        the plurality of missions being associated with a respective        per-mission nvPM impact parameter determined according to a        proposed fuel usage for that mission, the fuel usage defining        how the fuel allocation for the respective mission is to be used        during that mission; and    -   a fleetwide fuel allocation determination module configured to        determine the fleetwide fuel allocation for the plurality of        missions based on the fleet-wide optimisation.

The non-default fuel may be associated with a level of nvPM productionwhich is less than that of the default fuel.

The non-default fuel may be formed from a mixture of a first fuel havinga first fuel characteristic and a second fuel having a second fuelcharacteristic, different from the first.

The first and second fuel characteristics may be a percentage of SAFwithin the respective fuel, and wherein the non-default fuel is aSAF-rich fuel and the default fuel may be a relatively SAF-poor fuel.

The fleetwide optimisation module may be configured to perform thefollowing steps:

-   -   i) perform an outer-loop optimisation in which the fuel        allocation of one or more of the plurality of missions is varied        to reduce the sum of the per-mission nvPM impact parameters of        the plurality of missions; and    -   ii) perform an inner-loop optimisation in which the fuel usage        for each of the plurality missions is obtained according to the        constraints of the varied fuel allocation to determine a new        proposed fuel usage for each of the plurality of missions.

The fleetwide optimisation module may be configured to repeat steps i)and ii) until an optimised fuel usage for each of the plurality ofmissions is determined which corresponds to a minimised sum of theper-mission nvPM impact parameters.

The fleetwide optimisation module may be configured to perform theinner-loop optimisation by obtaining a pre-prepared solution for thefuel usage for a respective mission.

The fleetwide optimisation module may be configured to obtain theproposed fuel allocation for each of the plurality of missions byobtaining an optimised fuel usage for the respective mission defininghow the fuel is to be used in order to minimise the per-mission nvPMimpact parameter for that mission.

The fleetwide optimisation module may be configured to obtain theoptimised fuel usage for each mission by performing a per-missionoptimisation, the per-mission optimisation optionally comprising, foreach respective mission:

-   -   determining a type and/or operational capabilities of a        combustor used by the respective aircraft used for the mission;    -   determining a total fuel requirement for the respective mission;    -   determining an amount of fuel required for each type of fuel        injector provided in the combustor for the respective mission        where more than one type of injector is provided;    -   determining the dependence of nvPM emissions for each mission        engine operating point of the mission using fuel having the        characteristics of the default fuel, non-default fuel, or a        mixture thereof; and    -   determining an optimised fuel usage which minimises the total        nvPM emissions for the respective mission.

Determining a type of combustor used by the aircraft may comprisedetermining whether the aircraft comprises a lean-burn staged combustoror a rich-burn combustor.

If the combustor is a lean-burn staged combustor having pilot and mainfuel injectors, determining an amount of fuel required for each type offuel injector may comprise:

-   -   determining an amount of fuel required for the pilot fuel        injectors during pilot-and-main operation; and/or    -   determining an amount of fuel required for the pilot fuel        injectors during pilot-only operation; and/or    -   determining an amount of fuel required for the pilot fuel        injectors operating within a threshold range of the operation at        fuel flow rates below that of the staging point.

The fleetwide optimisation module may be configured to base thefleetwide optimisation on:

-   -   a percentage of a first fuel having a first fuel characteristic        within the default fuel defining the lowest possible percentage        of fuel having the first fuel characteristic which can be used        for combustion; and/or    -   a percentage of the first fuel having the first fuel        characteristic within the non-default fuel defining the highest        possible percentage of fuel having the first fuel characteristic        which can be used for combustion; and/or    -   the quantity of non-default fuel available for the plurality of        missions.

The present disclosure may apply to any relevant configuration of gasturbine engine. Such a gas turbine engine may be, for example, aturbofan gas turbine engine, an open rotor gas turbine engine (in whichthe propeller is not surrounded by a nacelle), a turboprop engine or aturbojet engine. Any such engine may or may not be provided with anafterburner.

A gas turbine engine in accordance with any aspect of the presentdisclosure may comprise an engine core comprising a turbine, acombustor, a compressor, and a core shaft connecting the turbine to thecompressor. Such a gas turbine engine may comprise a fan (having fanblades). Such a fan may be located upstream of the engine core.Alternatively, in some examples, the gas turbine engine may comprise afan located downstream of the engine core, for example where the gasturbine engine is an open rotor or a turboprop engine (in which case thefan may be referred to as a propeller).

Where the gas turbine engine is an open rotor or a turboprop engine, thegas turbine engine may comprise two contra-rotating propeller stagesattached to and driven by a free power turbine via a shaft. Thepropellers may rotate in opposite senses so that one rotates clockwiseand the other anti-clockwise around the engine's rotational axis.Alternatively, the gas turbine engine may comprise a propeller stage anda guide vane stage configured downstream of the propeller stage. Theguide vane stage may be of variable pitch. Accordingly, high-pressure,intermediate pressure, and free power turbines respectively may drivehigh and intermediate pressure compressors and propellers by suitableinterconnecting shafts. Thus, the propellers may provide the majority ofthe propulsive thrust.

Where the gas turbine engine is an open rotor or a turboprop engine, oneor more of the propeller stages may be driven by a gearbox. The gearboxmay be of the type described herein.

An engine according to the present disclosure may be a turbofan engine.Such an engine may be a direct-drive turbofan engine in which the fan isdirectly connected to the fan drive turbine, for example without agearbox. In such a direct-drive turbofan engine, the fan may be said torotate at the same rotational speed as the fan-drive turbine.

An engine according to the present disclosure may be a geared turbofanengine. In such an arrangement, the engine has a fan that is driven viaa gearbox. Accordingly, such a gas turbine engine may comprise a gearboxthat receives an input from the core shaft and outputs drive to the fanso as to drive the fan at a lower rotational speed than the core shaft.The input to the gearbox may be directly from the core shaft, orindirectly from the core shaft, for example via a spur shaft and/orgear. The core shaft may rigidly connect the turbine and the compressor,such that the turbine and compressor rotate at the same speed (with thefan rotating at a lower speed).

The gas turbine engine as described and/or claimed herein may have anysuitable general architecture. For example, the gas turbine engine mayhave any desired number of shafts that connect turbines and compressors,for example one, two or three shafts. Purely by way of example, theturbine connected to the core shaft may be a first turbine, thecompressor connected to the core shaft may be a first compressor, andthe core shaft may be a first core shaft. The engine core may furthercomprise a second turbine, a second compressor, and a second core shaftconnecting the second turbine to the second compressor. The secondturbine, second compressor, and second core shaft may be arranged torotate at a higher rotational speed than the first core shaft.

In such an arrangement, the second compressor may be positioned axiallydownstream of the first compressor. The second compressor may bearranged to receive (for example directly receive, for example via agenerally annular duct) flow from the first compressor.

The gearbox may be arranged to be driven by the core shaft that isconfigured to rotate (for example in use) at the lowest rotational speed(for example the first core shaft in the example above). For example,the gearbox may be arranged to be driven only by the core shaft that isconfigured to rotate (for example in use) at the lowest rotational speed(for example only be the first core shaft, and not the second coreshaft, in the example above). Alternatively, the gearbox may be arrangedto be driven by any one or more shafts, for example the first and/orsecond shafts in the example above.

The gearbox may be a reduction gearbox (in that the output to the fan isa lower rotational rate than the input from the core shaft). Any type ofgearbox may be used. For example, the gearbox may be a “planetary” or“star” gearbox, as described in more detail elsewhere herein. Such agearbox may be a single stage. Alternatively, such a gearbox may be acompound gearbox, for example a compound planetary gearbox (which mayhave the input on the sun gear and the output on the ring gear, and thusbe referred to as a “compound star” gearbox), for example having twostages of reduction.

The gearbox may have any desired reduction ratio (defined as therotational speed of the input shaft divided by the rotational speed ofthe output shaft), for example greater than 2.5, for example in therange of from 3 to 4.2, or 3.2 to 3.8, for example on the order of or atleast 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1 or 4.2. Thegear ratio may be, for example, between any two of the values in theprevious sentence. Purely by way of example, the gearbox may be a “star”gearbox having a reduction ratio in the range of from 3.1 or 3.2 to 3.8.Purely by way of further example, the gearbox may be a “star” gearboxhaving a reduction ratio in the range 3.0 to 3.1. Purely by way offurther example, the gearbox may be a “planetary” gearbox having areduction ratio in the range 3.6 to 4.2. In some arrangements, the gearratio may be outside these ranges.

In any gas turbine engine as described and/or claimed herein, fuel of agiven composition or blend is provided to a combustor, which may beprovided downstream of the fan and compressor(s) with respect to theflowpath (for example axially downstream). For example, the combustormay be directly downstream of (for example at the exit of) the secondcompressor, where a second compressor is provided. By way of furtherexample, the flow at the exit to the combustor may be provided to theinlet of the second turbine, where a second turbine is provided. Thecombustor may be provided upstream of the turbine(s).

The or each compressor (for example the first compressor and secondcompressor as described above) may comprise any number of stages, forexample multiple stages. Each stage may comprise a row of rotor bladesand a row of stator vanes, which may be variable stator vanes (in thattheir angle of incidence may be variable). The row of rotor blades andthe row of stator vanes may be axially offset from each other. Forexample, the gas turbine engine may be a direct-drive turbofan gasturbine engine comprising 13 or 14 compressor stages (in addition to thefan). Such an engine may, for example, comprise 3 stages in the first(or “low pressure”) compressor and either 10 or 11 stages in the second(or “high pressure”) compressor. By way of further example, the gasturbine engine may be a “geared” gas turbine engine (in which the fan isdrive by a first core shaft via a reduction gearbox) comprising 11, 12or 13 compressor stages (in addition to the fan). Such an engine maycomprise 3 or 4 stages in the first (or “low pressure”) compressor and 8or 9 stages in the second (or “high pressure”) compressor. By way offurther example, the gas turbine engine may be a “geared” gas turbineengine having 4 stages in the first (or “low pressure”) compressor and10 stages in the second (or “high pressure”) compressor.

The or each turbine (for example the first turbine and second turbine asdescribed above) may comprise any number of stages, for example multiplestages. Each stage may comprise a row of rotor blades and a row ofstator vanes. The row of rotor blades and the row of stator vanes may beaxially offset from each other. The second (or “high pressure”) turbinemay comprise 2 stages in any arrangement (for example regardless ofwhether it is a geared or direct-drive engine). The gas turbine enginemay be a direct-drive gas turbine engine comprising a first (or “lowpressure”) turbine having 5, 6 or 7 stages. Alternatively, the gasturbine engine may be a “geared” gas turbine engine comprising a first(or “low pressure”) turbine having 3 or 4 stages.

Each fan blade may be defined as having a radial span extending from aroot (or hub) at a radially inner gas-washed location, or 0% spanposition, to a tip at a 100% span position. The ratio of the radius ofthe fan blade at the hub to the radius of the fan blade at the tip maybe less than (or on the order of) any of: 0.4, 0.39, 0.38, 0.37, 0.36,0.35, 0.34, 0.33, 0.32, 0.31, 0.3, 0.29, 0.28, 0.27, 0.26, or 0.25. Theratio of the radius of the fan blade at the hub to the radius of the fanblade at the tip may be in an inclusive range bounded by any two of thevalues in the previous sentence (i.e. the values may form upper or lowerbounds), for example in the range of from 0.28 to 0.32 or 0.29 to 0.30.These ratios may commonly be referred to as the hub-to-tip ratio. Theradius at the hub and the radius at the tip may both be measured at theleading edge (or axially forwardmost) part of the blade. The hub-to-tipratio refers, of course, to the gas-washed portion of the fan blade,i.e. the portion radially outside any platform.

The radius of the fan may be measured between the engine centreline andthe tip of a fan blade at its leading edge. The fan diameter (which maysimply be twice the radius of the fan) may be greater than (or on theorder of) any of: 140 cm, 170 cm, 180 cm, 190 cm, 200 cm, 210 cm, 220cm, 230 cm, 240 cm, 250 cm (around 100 inches), 260 cm, 270 cm (around105 inches), 280 cm (around 110 inches), 290 cm (around 115 inches), 300cm (around 120 inches), 310 cm, 320 cm (around 125 inches), 330 cm(around 130 inches), 340 cm (around 135 inches), 350 cm, 360 cm (around140 inches), 370 cm (around 145 inches), 380 (around 150 inches) cm, 390cm (around 155 inches), 400 cm, 410 cm (around 160 inches) or 420 cm(around 165 inches). The fan diameter may be in an inclusive rangebounded by any two of the values in the previous sentence (i.e. thevalues may form upper or lower bounds), for example in the range of from240 cm to 280 cm or 330 cm to 380 cm. Purely by way of non-limitativeexample, the fan diameter may be in the range of from 170 cm to 180 cm,190 cm to 200 cm, 200 cm to 210 cm, 210 cm to 230 cm, 290 cm to 300 cmor 340 cm to 360 cm.

The rotational speed of the fan may vary in use. Generally, therotational speed is lower for fans with a higher diameter. Purely by wayof non-limitative example, the rotational speed of the fan at cruiseconditions may be less than 3500 rpm, for example less than 2500 rpm,for example less than 2300 rpm. Purely by way of further non-limitativeexample, the rotational speed of the fan at cruise conditions for an“geared” gas turbine engine having a fan diameter in the range of from200 cm to 210 cm may be in the range of from 2750 to 2900 rpm. Purely byway of further non-limitative example, the rotational speed of the fanat cruise conditions for an “geared” gas turbine engine having a fandiameter in the range of from 210 cm to 230 cm may be in the range offrom 2500 to 2800 rpm. Purely by way of further non-limitative example,the rotational speed of the fan at cruise conditions for an “geared” gasturbine engine having a fan diameter in the range of from 340 cm to 360cm may be in the range of from 1500 to 1800 rpm. Purely by way offurther non-limitative example, the rotational speed of the fan atcruise conditions for a direct drive engine having a fan diameter in therange of from 190 cm to 200 cm may be in the range of from 3600 to 3900rpm. Purely by way of further non-limitative example, the rotationalspeed of the fan at cruise conditions for a direct drive engine having afan diameter in the range of from 300 cm to 340 cm may be in the rangeof from 2000 to 2800 rpm.

In use of the gas turbine engine, the fan (with associated fan blades)rotates about a rotational axis. This rotation results in the tip of thefan blade moving with a velocity U_(tip). The work done by the fanblades 23 on the flow results in an enthalpy rise dH of the flow. A fantip loading may be defined as dH/U_(tip) ², where dH is the enthalpyrise (for example the 1-D average enthalpy rise) across the fan andU_(tip) is the (translational) velocity of the fan tip, for example atthe leading edge of the tip (which may be defined as fan tip radius atleading edge multiplied by angular speed). The fan tip loading at cruiseconditions may be greater than (or on the order of) any of: 0.28, 0.29,0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39 or 0.4 (allvalues being dimensionless). The fan tip loading may be in an inclusiverange bounded by any two of the values in the previous sentence (i.e.the values may form upper or lower bounds), for example in the range offrom 0.28 to 0.31, or 0.29 to 0.3 (for example for a geared gas turbineengine).

Gas turbine engines in accordance with the present disclosure may haveany desired bypass ratio, where the bypass ratio is defined as the ratioof the mass flow rate of the flow through the bypass duct to the massflow rate of the flow through the core at cruise conditions. In somearrangements the bypass ratio may be greater than (or on the order of)any of the following: 9. 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5,14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5 or 20. Thebypass ratio may be in an inclusive range bounded by any two of thevalues in the previous sentence (i.e. the values may form upper or lowerbounds), for example in the range of from 12 to 16, 13 to 15, or 13 to14. Purely by way of non-limitative example, the bypass ratio of adirect-drive gas turbine engine according to the present disclosure maybe in the range of from 9:1 to 11:1. Purely by way of furthernon-limitative example, the bypass ratio of a geared gas turbine engineaccording to the present disclosure may be in the range of from 12:1 to15:1 The bypass duct may be substantially annular. The bypass duct maybe radially outside the core engine. The radially outer surface of thebypass duct may be defined by a nacelle and/or a fan case.

The overall pressure ratio of a gas turbine engine as described and/orclaimed herein may be defined as the ratio of the stagnation pressure atthe exit of the highest pressure compressor (before entry into thecombustor) to the stagnation pressure upstream of the fan. By way ofnon-limitative example, the overall pressure ratio of a gas turbineengine as described and/or claimed herein at cruise may be greater than(or on the order of) any of the following: 35, 40, 45, 50, 55, 60, 65,70, 75. The overall pressure ratio may be in an inclusive range boundedby any two of the values in the previous sentence (i.e. the values mayform upper or lower bounds), for example in the range of from 50 to 70.Purely by way of non-limitative example, the overall pressure ratio atcruise conditions of a geared gas turbine engine having a fan diameterin the range of from 200 cm to 210 cm may be in the range of from 40 to45. Purely by way of non-limitative example, the overall pressure ratioat cruise conditions of a geared gas turbine engine having a fandiameter in the range of from 210 cm to 230 cm may be in the range offrom 45 to 55. Purely by way of non-limitative example, the overallpressure ratio at cruise conditions of a geared gas turbine enginehaving a fan diameter in the range of from 340 cm to 360 cm may be inthe range of from 50 to 60. Purely by way of non-limitative example, theoverall pressure ratio at cruise conditions of a direct-drive gasturbine engine having a fan diameter in the range of from 300 cm to 340cm may be in the range of from 50 to 60.

Specific thrust of an engine may be defined as the net thrust of theengine divided by the total mass flow through the engine. In someexamples, specific thrust may depend, for a given thrust condition, uponthe specific composition of fuel provided to the combustor. At cruiseconditions, the specific thrust of an engine described and/or claimedherein may be less than (or on the order of) any of the following: 110Nkg⁻¹s, 105 Nkg⁻¹s, 100 Nkg⁻¹s, 95 Nkg⁻¹s, 90 Nkg⁻¹s, 85 Nkg⁻¹s or 80Nkg⁻¹s. The specific thrust may be in an inclusive range bounded by anytwo of the values in the previous sentence (i.e. the values may formupper or lower bounds), for example in the range of from 80 Nkg⁻¹s to100 Nkg⁻¹s, or 85 Nkg⁻¹s to 95 Nkg⁻¹s. Such engines may be particularlyefficient in comparison with conventional gas turbine engines. Purely byway of non-limitative example, the specific thrust of a geared gasturbine engine having a fan diameter in the range of from 200 cm to 210cm may be in the range of from 90 Nkg⁻¹s to 95 Nkg⁻¹s. Purely by way ofnon-limitative example, the specific thrust of a geared gas turbineengine having a fan diameter in the range of from 210 cm to 230 cm maybe in the range of from 80 Nkg⁻¹s to 90 Nkg⁻¹s. Purely by way ofnon-limitative example, the specific thrust of a geared gas turbineengine having a fan diameter in the range of from 340 cm to 360 cm maybe in the range of from 70 Nkg⁻¹s to 90 Nkg⁻¹s. Purely by way ofnon-limitative example, the specific thrust of a direct drive gasturbine engine having a fan diameter in the range of from 300 cm to 340cm may be in the range of from 90 Nkg⁻¹s to 120 Nkg⁻¹s.

A gas turbine engine as described and/or claimed herein may have anydesired maximum thrust. Purely by way of non-limitative example, a gasturbine as described and/or claimed herein may be capable of producing amaximum thrust of at least (or on the order of) any of the following:100 kN, 110 kN, 120 kN, 130 kN, 140 kN, 150 kN, 160 kN, 170 kN, 180 kN,190 kN, 200 kN, 250 kN, 300 kN, 350 kN, 400 kN, 450 kN, 500 kN, or 550kN. The maximum thrust may be in an inclusive range bounded by any twoof the values in the previous sentence (i.e. the values may form upperor lower bounds). Purely by way of non-limitative example, a gas turbineas described and/or claimed herein may be capable of producing a maximumthrust in the range of from 330 kN to 420 kN, for example 350 kN to 400kN. Purely by way of non-limitative example, the maximum thrust of ageared gas turbine engine having a fan diameter in the range of from 200cm to 210 cm may be in the range of from 140 kN to 160 kN. Purely by wayof non-limitative example, the maximum thrust of a geared gas turbineengine having a fan diameter in the range of from 210 cm to 230 cm maybe in the range of from 150 kN to 200 kN. Purely by way ofnon-limitative example, the maximum thrust of a geared gas turbineengine having a fan diameter in the range of from 340 cm to 360 cm maybe in the range of from 370 kN to 500 kN. Purely by way ofnon-limitative example, the maximum thrust of a direct drive gas turbineengine having a fan diameter in the range of from 300 cm to 340 cm maybe in the range of from 370 kN to 500 kN. The thrust referred to abovemay be the maximum net thrust at standard atmospheric conditions at sealevel plus 15 degrees C. (ambient pressure 101.3 kPa, temperature 30degrees C.), with the engine static.

In use, the temperature of the flow at the entry to the high pressureturbine may be particularly high. This temperature, which may bereferred to as TET, may be measured at the exit to the combustor, forexample immediately upstream of the first turbine vane, which itself maybe referred to as a nozzle guide vane. In some examples, TET may depend,for a given thrust condition, upon the specific composition of fuelprovided to the combustor. At cruise, the TET may be at least (or on theorder of) any of the following: 1400K, 1450K, 1500K, 1550K, 1600K or1650K. Thus, purely by way of non-limitative example, the TET at cruiseof a geared gas turbine engine having a fan diameter in the range offrom 200 cm to 210 cm may be in the range of from 1540K to 1600K. Purelyby way of non-limitative example, the TET at cruise of a geared gasturbine engine having a fan diameter in the range of from 210 cm to 230cm may be in the range of from 1590K to 1650K. Purely by way ofnon-limitative example, the TET at cruise of a geared gas turbine enginehaving a fan diameter in the range of from 340 cm to 360 cm may be inthe range of from 1600K to 1660K. Purely by way of non-limitativeexample, the TET at cruise of a direct drive gas turbine engine having afan diameter in the range of from 300 cm to 340 cm may be in the rangeof from 1590K to 1650K. Purely by way of non-limitative example, the TETat cruise of a direct drive gas turbine engine having a fan diameter inthe range of from 300 cm to 340 cm may be in the range of from 1570K to1630K.

The TET at cruise may be in an inclusive range bounded by any two of thevalues in the previous sentence (i.e. the values may form upper or lowerbounds). The maximum TET in use of the engine may be, for example, atleast (or on the order of) any of the following: 1700K, 1750K, 1800K,1850K, 1900K, 1950K, 2000K, 2050K, or 2100K. Thus, purely by way ofnon-limitative example, the maximum TET of a geared gas turbine enginehaving a fan diameter in the range of from 200 cm to 210 cm may be inthe range of from 1890K to 1960K. Purely by way of non-limitativeexample, the maximum TET of a geared gas turbine engine having a fandiameter in the range of from 210 cm to 230 cm may be in the range offrom 1890K to 1960K. Purely by way of non-limitative example, themaximum TET of a geared gas turbine engine having a fan diameter in therange of from 340 cm to 360 cm may be in the range of from 1890K to1960K. Purely by way of non-limitative example, the maximum TET of adirect drive gas turbine engine having a fan diameter in the range offrom 300 cm to 340 cm may be in the range of from 1935K to 1995K. Purelyby way of non-limitative example, the maximum TET of a direct drive gasturbine engine having a fan diameter in the range of from 300 cm to 340cm may be in the range of from 1890K to 1950K. The maximum TET may be inan inclusive range bounded by any two of the values in the previoussentence (i.e. the values may form upper or lower bounds), for examplein the range of from 1800K to 1950K. The maximum TET may occur, forexample, at a high thrust condition, for example at a maximum take-off(MTO) condition.

A fan blade and/or aerofoil portion of a fan blade described and/orclaimed herein may be manufactured from any suitable material orcombination of materials. For example at least a part of the fan bladeand/or aerofoil may be manufactured at least in part from a composite,for example a metal matrix composite and/or an organic matrix composite,such as carbon fibre composite. By way of further example at least apart of the fan blade and/or aerofoil may be manufactured at least inpart from a metal, such as a titanium based metal or an aluminium basedmaterial (such as an aluminium-lithium alloy) or a steel based material.The fan blade may comprise at least two regions manufactured usingdifferent materials. For example, the fan blade may have a protectiveleading edge, which may be manufactured using a material that is betterable to resist impact (for example from birds, ice or other material)than the rest of the blade. Such a leading edge may, for example, bemanufactured using titanium or a titanium-based alloy. Thus, purely byway of example, the fan blade may have a carbon-fibre or aluminium basedbody (such as an aluminium lithium alloy) with a titanium leading edge.

A fan as described and/or claimed herein may comprise a central portion,from which the fan blades may extend, for example in a radial direction.The fan blades may be attached to the central portion in any desiredmanner. For example, each fan blade may comprise a fixture which mayengage a corresponding slot in the hub (or disc). Purely by way ofexample, such a fixture may be in the form of a dovetail that may slotinto and/or engage a corresponding slot in the hub/disc in order to fixthe fan blade to the hub/disc. By way of further example, the fan bladesmay be formed integrally with a central portion. Such an arrangement maybe referred to as a bladed disc or a bladed ring. Any suitable methodmay be used to manufacture such a bladed disc or bladed ring. Forexample, at least a part of the fan blades may be machined from a blockand/or at least part of the fan blades may be attached to the hub/discby welding, such as linear friction welding.

The gas turbine engines described and/or claimed herein may or may notbe provided with a variable area nozzle (VAN). Such a variable areanozzle may allow the exit area of the bypass duct to be varied in use.The general principles of the present disclosure may apply to engineswith or without a VAN.

The fan of a gas turbine as described and/or claimed herein may have anydesired number of fan blades, for example 14, 16, 18, 20, 22, 24 or 26fan blades. Where the fan blades have a carbon fibre composite body,there may be 16 or 18 fan blades. Where the fan blades have a metallicbody (for example aluminium-lithium or titanium-alloy), there may be 18,20 or 22 fan blades.

As used herein, the terms idle, taxi, take-off, climb, cruise, descent,approach, and landing have the conventional meaning and would be readilyunderstood by the skilled person. Thus, for a given gas turbine enginefor an aircraft, the skilled person would immediately recognise eachterm to refer to an operating phase of the engine within a given missionof an aircraft to which the gas turbine engine is designed to beattached.

In this regard, ground idle may refer to an operating phase of theengine where the aircraft is stationary and in contact with the ground,but where there is a requirement for the engine to be running. Duringidle, the engine may be producing between 3% and 9% of the availablethrust of the engine. In further non-limitative examples, the engine maybe producing between 5% and 8% of available thrust. In furthernon-limitative examples, the engine may be producing between 6% and 7%of available thrust. Taxi may refer to an operating phase of the enginewhere the aircraft is being propelled along the ground by the thrustproduced by the engine. During taxi, the engine may be producing between5% and 15% of available thrust. In further non-limitative examples, theengine may be producing between 6% and 12% of available thrust. Infurther non-limitative examples, the engine may be producing between 7%and 10% of available thrust. Take-off may refer to an operating phase ofthe engine where the aircraft is being propelled by the thrust producedby the engine. At an initial stage within the take-off phase, theaircraft may be propelled whilst the aircraft is in contact with theground. At a later stage within the take-off phase, the aircraft may bepropelled whilst the aircraft is not in contact with the ground. Duringtake-off, the engine may be producing between 90% and 100% of availablethrust. In further non-limitative examples, the engine may be producingbetween 95% and 100% of available thrust. In further non-limitativeexamples, the engine may be producing 100% of available thrust.

Climb may refer to an operating phase of the engine where the aircraftis being propelled by the thrust produced by the engine. During climb,the engine may be producing between 75% and 100% of available thrust. Infurther non-limitative examples, the engine may be producing between 80%and 95% of available thrust. In further non-limitative examples, theengine may be producing between 85% and 90% of available thrust. In thisregard, climb may refer to an operating phase within an aircraft flightcycle between take-off and the arrival at cruise conditions.Additionally or alternatively, climb may refer to a nominal point in anaircraft flight cycle between take-off and landing, where a relativeincrease in altitude is required, which may require an additional thrustdemand of the engine.

As used herein, cruise conditions have the conventional meaning andwould be readily understood by the skilled person. Thus, for a given gasturbine engine for an aircraft, the skilled person would immediatelyrecognise cruise conditions to mean the operating point of the engine atmid-cruise of a given mission (which may be referred to in the industryas the “economic mission”) of an aircraft to which the gas turbineengine is designed to be attached. In this regard, mid-cruise is thepoint in an aircraft flight cycle at which 50% of the total fuel that isburned between top of climb and start of descent has been burned (whichmay be approximated by the midpoint—in terms of time and/ordistance—between top of climb and start of descent. Cruise conditionsthus define an operating point of the gas turbine engine that provides athrust that would ensure steady state operation (i.e. maintaining aconstant altitude and constant Mach Number) at mid-cruise of an aircraftto which it is designed to be attached, taking into account the numberof engines provided to that aircraft. For example where an engine isdesigned to be attached to an aircraft that has two engines of the sametype, at cruise conditions the engine provides half of the total thrustthat would be required for steady state operation of that aircraft atmid-cruise.

In other words, for a given gas turbine engine for an aircraft, cruiseconditions are defined as the operating point of the engine thatprovides a specified thrust (required to provide—in combination with anyother engines on the aircraft—steady state operation of the aircraft towhich it is designed to be attached at a given mid-cruise Mach Number)at the mid-cruise atmospheric conditions (defined by the InternationalStandard Atmosphere according to ISO 2533 at the mid-cruise altitude).For any given gas turbine engine for an aircraft, the mid-cruise thrust,atmospheric conditions and Mach Number are known, and thus the operatingpoint of the engine at cruise conditions is clearly defined.

Purely by way of example, the forward speed at the cruise condition maybe any point in the range of from Mach 0.7 to 0.9, for example 0.75 to0.85, for example 0.76 to 0.84, for example 0.77 to 0.83, for example0.78 to 0.82, for example 0.79 to 0.81, for example on the order of Mach0.8, on the order of Mach 0.85 or in the range of from 0.8 to 0.85. Anysingle speed within these ranges may be part of the cruise condition.For some aircraft, the cruise conditions may be outside these ranges,for example below Mach 0.7 or above Mach 0.9.

Purely by way of example, the cruise conditions may correspond tostandard atmospheric conditions (according to the International StandardAtmosphere, ISA) at an altitude that is in the range of from 10000 m to15000 m, for example in the range of from 10000 m to 12000 m, forexample in the range of from 10400 m to 11600 m (around 38000 ft), forexample in the range of from 10500 m to 11500 m, for example in therange of from 10600 m to 11400 m, for example in the range of from 10700m (around 35000 ft) to 11300 m, for example in the range of from 10800 mto 11200 m, for example in the range of from 10900 m to 11100 m, forexample on the order of 11000 m. The cruise conditions may correspond tostandard atmospheric conditions at any given altitude in these ranges.

Purely by way of example, the cruise conditions may correspond to anoperating point of the engine that provides a known required thrustlevel (for example a value in the range of from 30 kN to 35 kN) at aforward Mach number of 0.8 and standard atmospheric conditions(according to the International Standard Atmosphere) at an altitude of38000 ft (11582 m). Purely by way of further example, the cruiseconditions may correspond to an operating point of the engine thatprovides a known required thrust level (for example a value in the rangeof from 50 kN to 65 kN) at a forward Mach number of 0.85 and standardatmospheric conditions (according to the International StandardAtmosphere) at an altitude of 35000 ft (10668 m).

In use, a gas turbine engine described and/or claimed herein may operateat the cruise conditions defined elsewhere herein. Such cruiseconditions may be determined by the cruise conditions (for example themid-cruise conditions) of an aircraft to which at least one (for example2 or 4) gas turbine engine may be mounted in order to provide propulsivethrust.

Furthermore, the skilled person would immediately recognise either orboth of descent and approach to refer to an operating phase within anaircraft flight cycle between cruise and landing of the aircraft. Duringeither or both of descent and approach, the engine may be producingbetween 20% and 50% of available thrust. In further non-limitativeexamples, the engine may be producing between 25% and 40% of availablethrust. In further non-limitative examples, the engine may be producingbetween 30% and 35% of available thrust. Additionally or alternatively,descent may refer to a nominal point in an aircraft flight cycle betweentake-off and landing, where a relative decrease in altitude is required,and which may require a reduced thrust demand of the engine.

According to an aspect, there is provided an aircraft comprising a gasturbine engine as described and/or claimed herein. The aircraftaccording to this aspect is the aircraft for which the gas turbineengine has been designed to be attached. Accordingly, the cruiseconditions according to this aspect correspond to the mid-cruise of theaircraft, as defined elsewhere herein.

According to an aspect, there is provided a method of operating a gasturbine engine as described and/or claimed herein. The operation may beat any suitable condition, which may be as defined elsewhere herein (forexample in terms of the thrust, atmospheric conditions and Mach Number).

According to an aspect, there is provided a method of operating anaircraft comprising a gas turbine engine as described and/or claimedherein. The operation according to this aspect may include (or may be)operation at any suitable condition, for example the mid-cruise of theaircraft, as defined elsewhere herein.

The skilled person will appreciate that except where mutually exclusive,a feature or parameter described in relation to any one of the aboveaspects may be applied to any other aspect. Furthermore, except wheremutually exclusive, any feature or parameter described herein may beapplied to any aspect and/or combined with any other feature orparameter described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only, with referenceto the Figures, in which:

FIG. 1 is a sectional side view of a gas turbine engine;

FIG. 2 is a close up sectional side view of an upstream portion of a gasturbine engine;

FIG. 3 is a partially cut-away view of a gearbox for a gas turbineengine;

FIG. 4 is a schematic view of an aircraft having two fuel sources;

FIG. 5 is a schematic view of a combustion system of a gas turbineengine connected to two fuel sources;

FIG. 6 illustrates a relationship between fuel flow rate and nvPMnumber;

FIG. 7 illustrates nvPM number dependencies upon fuel flow rate forvarious fuel compositions;

FIG. 8 illustrates an alternative nvPM number dependency upon fuel flowrate for various fuel compositions;

FIG. 9 shows a schematic view of another combustion system;

FIG. 10 illustrates the dependency of nvPM number upon fuel flow ratefor the arrangement of FIG. 9 ;

FIG. 11 illustrates a method of operating a gas turbine engine;

FIG. 12 shows a schematic view of another combustion system;

FIG. 13 illustrates the dependency of nvPM number upon fuel flow ratefor the arrangement of FIG. 12 ;

FIG. 14 illustrates another method of operating a gas turbine engine;

FIG. 15 illustrates another example of the dependency of nvPM numberupon fuel flow rate for the arrangement of FIG. 12 ;

FIG. 16 illustrates another method of operating a gas turbine engine;

FIG. 17 shows a schematic view of another combustion system;

FIG. 18 illustrates the dependency of nvPM number upon fuel flow ratefor the arrangement shown in FIG. 17 ;

FIG. 19 illustrates a region of feasible nvPM numbers in dependence uponfuel flow rate for the arrangement shown in FIG. 17 ;

FIG. 20 illustrates another dependency of nvPM number upon fuel flowrate for the arrangement shown in FIG. 17 ;

FIG. 21 illustrates another method of operating a gas turbine engine;

FIG. 22 illustrates the dependence of the carbon monoxide (CO) emissionindex (EI) and unburnt hydrocarbon (HC) emission index on engine powersetting;

FIG. 23 illustrates the dependence of carbon monoxide (CO) emissionindex (EI) and unburnt hydrocarbon (HC) emission index on engine powersetting with a staging point (SP) set to define first and second cruiseoperation ranges;

FIG. 24 illustrates the dependence of nvPM emission index on enginepower setting;

FIG. 25 illustrates the dependence of nvPM emission index on enginepower setting to illustrate the result of selectively using fuel of adifferent characteristic during a first engine cruise operation;

FIG. 26 illustrates another method of operating a gas turbine engine;

FIG. 27 illustrates the dependence of CO and HC emission index (EI) onengine power setting to illustrate the effect of a transition range ofoperation between a pilot-only and pilot-and-main range of operation;

FIG. 28 illustrates the dependence of nvPM emission index on enginepower setting to further illustrate the effect of the transition rangeof operation;

FIG. 29 illustrates another method of operating a gas turbine engine;

FIG. 30 illustrates another method of operating a gas turbine engine;

FIG. 31 shows a schematic view of another combustion system;

FIG. 32 illustrates another method of operating a gas turbine engine;

FIG. 33 illustrates another method of operating a gas turbine engine;

FIG. 34 illustrates a method of determining one or more fuel loadingparameters;

FIG. 35 illustrates a method of loading fuel onto an aircraft;

FIG. 36 shows a schematic view of a fuel loading parameter determinationsystem;

FIG. 37 illustrates a method of determining an optimised fleetwide fuelallocation for a plurality of flights;

FIG. 38 illustrates a per-flight optimisation to determine an optimisedfuel usage;

FIG. 39 illustrates a method of loading fuel onto a plurality ofaircraft;

FIG. 40 shows a schematic view of a fleetwide fuel allocationdetermination system; and

FIG. 41 shows a schematic view of a computing device.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 1 illustrates a gas turbine engine 10 having a principal rotationalaxis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23that generates two airflows: a core airflow A and a bypass airflow B.The gas turbine engine 10 comprises a core 11 that receives the coreairflow A. The engine core 11 comprises, in axial flow series, a lowpressure compressor 14, a high-pressure compressor 15, combustionequipment 16, a high-pressure turbine 17, a low pressure turbine 19 anda core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. Thebypass airflow B flows through the bypass duct 22. The fan 23 isattached to and driven by the low pressure turbine 19 via a shaft 26 andan epicyclic gearbox 30.

In use, the core airflow A is accelerated and compressed by the lowpressure compressor 14 and directed into the high pressure compressor 15where further compression takes place. The compressed air exhausted fromthe high pressure compressor 15 is directed into the combustionequipment 16 where it is mixed with fuel F and the mixture is combusted.The combustion equipment 16 may be referred to as the combustor 16, withthe terms “combustion equipment 16” and “combustor 16” usedinterchangeably herein. The resultant hot combustion products thenexpand through, and thereby drive, the high pressure and low pressureturbines 17, 19 before being exhausted through the nozzle 20 to providesome propulsive thrust. The high pressure turbine 17 drives the highpressure compressor 15 by a suitable interconnecting shaft 27. The fan23 generally provides the majority of the propulsive thrust. Theepicyclic gearbox 30 is a reduction gearbox.

An exemplary arrangement for a geared fan gas turbine engine 10 is shownin FIG. 2 . The low pressure turbine 19 (see FIG. 1 ) drives the shaft26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclicgear arrangement 30. Radially outwardly of the sun gear 28 andintermeshing therewith is a plurality of planet gears 32 that arecoupled together by a planet carrier 34. The planet carrier 34constrains the planet gears 32 to precess around the sun gear 28 insynchronicity whilst enabling each planet gear 32 to rotate about itsown axis. The planet carrier 34 is coupled via linkages 36 to the fan 23in order to drive its rotation about the engine axis 9. Radiallyoutwardly of the planet gears 32 and intermeshing therewith is anannulus or ring gear 38 that is coupled, via linkages 40, to astationary supporting structure 24.

Note that the terms “low pressure turbine” and “low pressure compressor”as used herein may be taken to mean the lowest pressure turbine stagesand lowest pressure compressor stages (i.e. not including the fan 23)respectively and/or the turbine and compressor stages that are connectedtogether by the interconnecting shaft 26 with the lowest rotationalspeed in the engine (i.e. not including the gearbox output shaft thatdrives the fan 23). In some literature, the “low pressure turbine” and“low pressure compressor” referred to herein may alternatively be knownas the “intermediate pressure turbine” and “intermediate pressurecompressor”. Where such alternative nomenclature is used, the fan 23 maybe referred to as a first, or lowest pressure, compression stage.

The epicyclic gearbox 30 is shown by way of example in greater detail inFIG. 3 . Each of the sun gear 28, planet gears 32 and ring gear 38comprise teeth about their periphery to intermesh with the other gears.However, for clarity only exemplary portions of the teeth areillustrated in FIG. 3 . There are four planet gears 32 illustrated,although it will be apparent to the skilled reader that more or fewerplanet gears 32 may be provided within the scope of the claimedinvention. Practical applications of a planetary epicyclic gearbox 30generally comprise at least three planet gears 32.

The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3is of the planetary type, in that the planet carrier 34 is coupled to anoutput shaft via linkages 36, with the ring gear 38 fixed. However, anyother suitable type of epicyclic gearbox 30 may be used. By way offurther example, the epicyclic gearbox 30 may be a star arrangement, inwhich the planet carrier 34 is held fixed, with the ring (or annulus)gear 38 allowed to rotate. In such an arrangement the fan 23 is drivenby the ring gear 38. By way of further alternative example, the gearbox30 may be a differential gearbox in which the ring gear 38 and theplanet carrier 34 are both allowed to rotate.

It will be appreciated that the arrangement shown in FIGS. 2 and 3 is byway of example only, and various alternatives are within the scope ofthe present disclosure. Purely by way of example, any suitablearrangement may be used for locating the gearbox 30 in the engine 10and/or for connecting the gearbox 30 to the engine 10. By way of furtherexample, the connections (such as the linkages 36, 40 in the FIG. 2example) between the gearbox 30 and other parts of the engine 10 (suchas the input shaft 26, the output shaft and the fixed structure 24) mayhave any desired degree of stiffness or flexibility. By way of furtherexample, any suitable arrangement of the bearings between rotating andstationary parts of the engine (for example between the input and outputshafts from the gearbox and the fixed structures, such as the gearboxcasing) may be used, and the disclosure is not limited to the exemplaryarrangement of FIG. 2 . For example, where the gearbox 30 has a stararrangement (described above), the skilled person would readilyunderstand that the arrangement of output and support linkages andbearing locations would typically be different to that shown by way ofexample in FIG. 2 .

Accordingly, the present disclosure extends to a gas turbine enginehaving any arrangement of gearbox styles (for example star orplanetary), support structures, input and output shaft arrangement, andbearing locations.

Optionally, the gearbox may drive additional and/or alternativecomponents (e.g. the intermediate pressure compressor and/or a boostercompressor).

Other gas turbine engines to which the present disclosure may be appliedmay have alternative configurations. For example, such engines may havean alternative number of compressors and/or turbines and/or analternative number of interconnecting shafts. By way of further example,the gas turbine engine shown in FIG. 1 has a split flow nozzle 18, 20meaning that the flow through the bypass duct 22 has its own nozzle 18that is separate to and radially outside the core engine nozzle 20.However, this is not limiting, and any aspect of the present disclosuremay also apply to engines in which the flow through the bypass duct 22and the flow through the core 11 are mixed, or combined, before (orupstream of) a single nozzle, which may be referred to as a mixed flownozzle. One or both nozzles (whether mixed or split flow) may have afixed or variable area.

Whilst the described example relates to a turbofan engine, thedisclosure may apply, for example, to any type of gas turbine engine,such as an open rotor (in which the fan stage is not surrounded by anacelle) or turboprop engine, for example. In some arrangements, the gasturbine engine 10 may not comprise a gearbox 30.

The geometry of the gas turbine engine 10, and components thereof, isdefined by a conventional axis system, comprising an axial direction(which is aligned with the rotational axis 9), a radial direction (inthe bottom-to-top direction in FIG. 1 ), and a circumferential direction(perpendicular to the page in the FIG. 1 view). The axial, radial andcircumferential directions are mutually perpendicular.

The fuel F provided to the combustion equipment 16 may comprise afossil-based hydrocarbon fuel, such as Kerosene. Thus, the fuel F maycomprise molecules from one or more of the chemical families ofn-alkanes, iso-alkanes, cycloalkanes, and aromatics. Additionally oralternatively, the fuel F may comprise renewable hydrocarbons producedfrom biological or non-biological resources, otherwise known assustainable aviation fuel (SAF). In each of the provided examples, thefuel F may comprise one or more trace elements including, for example,sulphur, nitrogen, oxygen, inorganics, and metals.

Functional performance of a given composition, or blend of fuel for usein a given mission, may be defined, at least in part, by the ability ofthe fuel to service the Brayton cycle of the gas turbine engine 10.Parameters defining functional performance may include, for example,specific energy; energy density; thermal stability; and, emissionsincluding particulate matter. A relatively higher specific energy (i.e.energy per unit mass), expressed as MJ/kg, may at least partially reducetake-off weight, thus potentially providing a relative improvement infuel efficiency. A relatively higher energy density (i.e. energy perunit volume), expressed as MJ/L, may at least partially reduce take-offfuel volume, which may be particularly important for volume-limitedmissions or military operations involving refuelling. A relativelyhigher thermal stability (i.e. inhibition of fuel to degrade or cokeunder thermal stress) may permit the fuel to sustain elevatedtemperatures in the engine and fuel injectors, thus potentiallyproviding relative improvements in combustion efficiency. Reducedemissions, including particulate matter, may permit reduced contrailformation, whilst reducing the environmental impact of a given mission.Other properties of the fuel may also be key to functional performance.For example, a relatively lower freeze point (° C.) may allow long-rangemissions to optimise flight profiles; minimum aromatic concentrations(%) may ensure sufficient swelling of certain materials used in theconstruction of o-rings and seals that have been previously exposed tofuels with high aromatic contents; and, a maximum surface tension (mN/m)may ensure sufficient spray break-up and atomisation of the fuel.

The ratio of the number of hydrogen atoms to the number of carbon atomsin a molecule may influence the specific energy of a given composition,or blend of fuel. Fuels with higher ratios of hydrogen atoms to carbonatoms may have higher specific energies in the absence of bond strain.For example, fossil-based hydrocarbon fuels may comprise molecules withapproximately 7 to 18 carbons, with a significant portion of a givencomposition stemming from molecules with 9 to 15 carbons, with anaverage of 12 carbons.

A number of sustainable aviation fuel blends have been approved for use,comprising between 10% and 50% sustainable aviation fuel (the remaindercomprising one or more fossil-based hydrocarbon fuels, such asKerosene), with further compositions awaiting approval. However, thereis an anticipation in the aviation industry that sustainable aviationfuel blends comprising up to (and including) 100% sustainable aviationfuel (SAF) will be eventually approved for use.

Sustainable aviation fuels may comprise one or more of n-alkanes,iso-alkanes, cyclo-alkanes, and aromatics, and may be produced, forexample, from one or more of synthesis gas (syngas); lipids (e.g. fats,oils, and greases); sugars; and alcohols. Thus, sustainable aviationfuels may comprise either or both of a lower aromatic and sulphurcontent, relative to fossil-based hydrocarbon fuels. Additionally oralternatively, sustainable aviation fuels may comprise either or both ofa higher iso-alkane and cyclo-alkane content, relative to fossil-basedhydrocarbon fuels. Thus, in some examples, sustainable aviation fuelsmay comprise either or both of a density of between 90% and 98% that ofkerosene and a calorific value of between 101% and 105% that ofkerosene.

Owing at least in part to the molecular structure of sustainableaviation fuels, sustainable aviation fuels may provide benefitsincluding, for example, one or more of a higher specific energy(despite, in some examples, a lower energy density); higher specificheat capacity; higher thermal stability; higher lubricity; lowerviscosity; lower surface tension; lower freeze point; lower sootemissions; and, lower CO₂ emissions, relative to fossil-basedhydrocarbon fuels (e.g. when combusted in the combustion equipment 16).Accordingly, relative to fossil-based hydrocarbon fuels, such asKerosene, sustainable aviation fuels may lead to either or both of arelative decrease in specific fuel consumption, and a relative decreasein maintenance costs.

Aircraft Fuel System and Fuel Characteristics

An aircraft 1 comprising two gas turbine engines 10 according to any ofthe examples described herein is illustrated in FIG. 4 . In thisexample, the aircraft 1 comprises two gas turbine engines 10, but inother examples may comprise one or more gas turbine engines. Theaircraft 1 further comprises an aircraft fuel system located on boardthe aircraft which is suitable for suppling fuel F to each of the gasturbine engines 10 to be burnt in the engine combustion equipment 16 asdescribed above. The aircraft fuel system is arranged to provide fuel toan engine fuel system provided on each of the gas turbine engines 10.The engine fuel system and aircraft fuel system together form the fuelsupply system of the aircraft 1.

The aircraft fuel system comprises two fuel sources, a first fuel source302 and a second fuel source 304 (given different shading in FIG. 4 ).For the purposes of the present application the term “fuel source” isunderstood to mean either 1) a single fuel tank or 2) a plurality offuel tanks which may or may not be fluidly interconnected. Each of thefuel sources are arranged to provide separate sources of fuel i.e. thefirst fuel source may contain a first fuel having a differentcharacteristic or characteristics to a second fuel contained in thesecond fuel source. The first and second fuel sources are therefore notfluidly coupled to each other so as to separate the different fuels (atleast under normal running conditions as discussed elsewhere herein).

The aircraft fuel system may comprise a plurality of fuel tanks that maybe selectively fluidly connected in different arrangements to form thefirst and second fuel sources. The fuel tanks may comprise associatedshut-off valves so that different combinations of tanks can be groupedtogether in different configurations. In such examples, there may bemultiple ways of assigning individual fuel tanks to the first fuelsource and the second fuel source. In other examples the assignment offuel tanks to each of the fuel sources may be fixed. In the presentexample, the total volume of fuel tanks forming the first fuel source302 is less than or equal to the volume of the fuel tanks forming thesecond fuel source 304. This may not however be the case for otherexamples.

In the present example, the first and second fuel sources 302, 304comprise a plurality of wing fuel tanks 53, where at least one wing fueltank is located in the port wing and at least one wing fuel tank islocated in the starboard wing, and a centre fuel tank 55 locatedprimarily in the fuselage of the aircraft 1. Each of the centre fueltank 55 and the wing fuel tanks 53 may comprise a plurality of fluidlyinterconnected fuel tanks not shown in the Figures. As shown by theshading in FIG. 4 , the wing fuel tanks 53 form the first fuel source302, with the second fuel source 304 being formed by the centre fueltank 55, in the present example.

For balancing purposes, one or more fuel tanks in the port wing may befluidly connected to one or more fuel tanks in the starboard wing. Thismay be done either via a centre fuel tank (if that tank does not formpart of the other fuel source), or bypassing the centre fuel tank(s), orboth (for maximum flexibility and safety).

In another example, the second fuel source 304 comprises wing fuel tanksand a centre fuel tank, while the first fuel source 302 comprises afurther separate centre fuel tank. Fluid interconnection between wingfuel tanks and the centre fuel tank of the second fuel source may beprovided for balancing of the aircraft.

In some examples, the allocation of fuel tanks available on the aircraftmay be constrained such that the first fuel source and the second fuelsource 302, 304 are each substantially symmetrical with respect to theaircraft centre line. In cases where an asymmetric fuel tank allocationis permitted, a suitable means of fuel transfer is provided between fueltanks of the first fuel source and/or between fuel tanks of the secondfuel source such that the position of the aircraft's centre of mass canbe maintained within acceptable lateral limits throughout the flight.

A trim fuel tank could in some examples be part of the first fuel source302, or in other examples part of the second fuel source 304. Theallocation of fuel tanks to the first fuel source 302 and the secondfuel source 304 may be constrained such that neither the first fuelsource 302 nor the second fuel source 304 comprises solely the trim fueltank.

While the examples described in the present application have only afirst and second fuel source 302, 304, in other examples further fuelsources may be provided so that fuel from any number of sources, eachhaving a different fuel characteristic or characteristics may besupplied to the combustor via the fuel delivery regulator.

The first and second fuel sources 302, 304 may be used to store fuelhaving different characteristics. This may allow fuel having differentcharacteristics to be provided to the combustion equipment 16 as will bedescribed in more detail in the various examples below.

As used herein, the term “fuel characteristics” refers to intrinsic orinherent fuel properties such as fuel composition, not variableproperties such as volume or temperature. Examples of fuelcharacteristics include one or more of:

-   -   i. the percentage of sustainable aviation fuel (SAF) in the        fuel, or an indication that the fuel is a fossil fuel, for        example fossil kerosene, or a pure SAF;    -   ii. parameters of a hydrocarbon distribution of the fuel, such        as:        -   the aromatic hydrocarbon content of the fuel, and optionally            also/alternatively the multi-aromatic hydrocarbon content of            the fuel;        -   the hydrogen to carbon ratio (H/C) of the fuel;        -   % composition information for some or all hydrocarbons            present;    -   iii. the presence or percentage of a particular element or        species, such as:        -   the percentage of nitrogen-containing species in the fuel;        -   the presence or percentage of a tracer species or trace            element in the fuel (e.g. a trace substance inherently            present in the fuel which may vary between fuels and so be            used to identify a fuel, and/or a substance added            deliberately to act as a tracer);        -   naphthalene content of the fuel;        -   sulphur content of the fuel;        -   cycloparaffin content of the fuel;        -   oxygen content of the fuel;    -   iv. one or more properties of the fuel in use in a gas turbine        engine 10, such as:        -   level of non-volatile particulate matter (nvPM) emissions or            CO₂ emissions on combustion (a value may be provided for a            specific combustor operating under particular conditions to            compare fuels fairly—a measured value may be adjusted            accordingly based on combustor properties and conditions);        -   level of coking of the fuel;    -   v. one or more properties of the fuel itself, independent of use        in an engine or combustion, such as:        -   thermal stability of the fuel (e.g. thermal breakdown            temperature); and        -   one or more physical properties such as density, viscosity,            calorific value, freeze temperature, and/or heat capacity.

The aircraft 1 may be refuelled by connecting a refuelling source 60,such as that provided by an airport fuel truck, fixed fuel storage, or apermanent pipeline, to a fuel line connection port 62 of the aircraft,via a fuel line 61. A desired amount of fuel may be transferred from therefuelling source 60 to the one or more tanks 53, 55 of the aircraft 1.In the present example in which different tanks 53, 55 are to be filledwith different fuels, multiple fuel line connection ports 62 areprovided. The refuelling source 60 therefore in this example holds twofuels, having different fuel characteristics, in separate vessels 60 a,60 b (e.g. a default and a non-default fuel as described later) inseparate tanks. In other examples, valves may be used to direct fuelappropriately if received from a single connection port.

Fuel characteristics may be obtained either by:

-   -   (i) physically and/or chemically detecting one or more        characteristics of a fuel, either during operation of the        aircraft (e.g. on wing), or as the aircraft is being refuelled;    -   (iii) receiving data, for example from an input provided at a        user interface, or data transmitted to the aircraft 1.

Fuel characteristics may be detected in various ways, both direct (e.g.from sensor data corresponding to the fuel characteristic in question)and indirect (e.g. by inference or calculation from othercharacteristics or measurements). The characteristics may be determinedas relative values as compared to another fuel, or as absolute values.For example, one or more of the following detection methods may be used:

-   -   The aromatic or cycloparaffin content of the fuel may be        determined based on measurements of the swell of a sensor        component made from a seal material such as a nitrile seal        material.    -   Trace substances or species, either present naturally in the        fuel or added to act as a tracer, may be used to determine fuel        characteristics such as the percentage of sustainable aviation        fuel in the fuel or whether the fuel is kerosene.    -   Measurements of the vibrational mode of a piezoelectric crystal        exposed to the fuel may be used as the basis for the        determination of various fuel characteristics including the        aromatic content of the fuel, the oxygen content of the fuel,        and the thermal stability or the coking level of the fuel—for        example by measuring the build-up of surface deposits on the        piezoelectric crystal which will result in a change in        vibrational mode.    -   Various fuel characteristics may be determined by collecting        performance parameters of the gas turbine engine 10 during a        first period of operation (such as during take-off) and then        during a second period of operation (e.g. during cruise)        comparing these collected parameters to expected values if using        fuel of known properties.    -   Various fuel characteristics including the aromatic hydrocarbon        content of the fuel may be determined based on sensor        measurements of the presence, absence, or degree of formation of        a contrail by the gas turbine 10 during its operation.    -   Fuel characteristics including the aromatic hydrocarbon content        may be determined based on a UV-Vis spectroscopy measurement        performed on the fuel.    -   Various fuel characteristics including the sulphur content,        naphthalene content, aromatic hydrogen content and hydrogen to        carbon ratio may be determined by measurement of substances        present in the exhaust gases emitted by the gas turbine engine        10 during its use.    -   Calorific value of the fuel may be determined in operation of        the aircraft 1 based on measurements taken as the fuel is being        burned—for example using fuel flow rate and shaft speed or        change in temperature across the combustor 16.    -   Various fuel characteristics may be determined by making an        operational change arranged to affect operation of the gas        turbine engine 10, sensing a response to the operational change;        and determining the one or more fuel characteristics of the fuel        based on the response to the operational change.    -   Various fuel characteristics may be determined in relation to        fuel characteristics of a first fuel by changing a fuel supplied        to the gas turbine engine 10 from the first fuel to a second        fuel, and determining the one or more fuel characteristics of        the second fuel based on a change in a relationship between T30        and one of T40 and T41 (the relationship being indicative of the        temperature rise across the combustor 16). The characteristics        may be determined as relative values as compared to the first        fuel, or as absolute values by reference to known values for the        first fuel.

As used herein and discussed below, T30, T40 and T41, and any othernumbered pressures and temperatures, are defined using the stationnumbering listed in standard SAE AS755, in particular:

-   -   T30=High Pressure Compressor (HPC) Outlet Temperature;    -   T40=Combustion Exit Temperature; and    -   T41=High Pressure Turbine (HPT) Rotor Entry Temperature.

Combustion System

FIG. 5 schematically illustrates further details of the combustionequipment 16 (which may be referred to simply as “the combustor”) of thegas turbine engine 10. In this example, the combustion equipment is partof a staged combustor system 64 in which fuel is injected via pilot andmain fuel injectors. Fuel is provided to the fuel injectors by means ofa fuel delivery regulator 306 under control of an electronic enginecontroller (EEC) 42. Fuel is delivered to the fuel delivery regulator byfuel pumps 308 a, 308 b. In the present example, fuel is provided to thefuel delivery regulator 306 from two different fuel sources (the firstfuel source 302 and the second fuel source 304) as described above. Fuelfrom each of the first and second fuel sources 302, 304 is provided by arespective first fuel pump 308 a and second fuel pump 308 b. Each ofthese fuel pumps may be mechanically driven by an accessory gearbox. Inalternative configurations, for example in a more electric engine (MEE)configuration, the fuel pump may be electrically-driven. The skilledperson will understand that any known suitable configuration of fuelpump or combination of fuel pumps may be used to deliver fuel from thefuel tanks 53, 55 to the combustor 16.

High-pressure fuel is delivered by the fuel delivery regulator 306 intoa pilot manifold 309 and a main manifold 310. In the present example,the staged combustion system comprises a plurality of fuel nozzles 311which are configured as duplex fuel injectors (also known asinternally-staged). In the present example, 16 circumferentially-spacedfuel nozzles are disposed around an annular combustor can 312. As willbe appreciated, more or indeed fewer fuel nozzles may be provideddepending upon the physical engine size and design requirements.Further, different combustor configurations may be used, for examplecannular, canned, etc.

Fuel from the pilot manifold 309 and the main manifold 310 is deliveredto all of the fuel nozzles 311 for injection into the combustor can 312.In the present example, a central pilot fuel injector 313 produces apilot fuel spray in a primary zone of the combustor can 312, whilst aconcentric main fuel injector 314 produces a main fuel spray.

Delivery of fuel via the pilot and main fuel injectors 313, 314 isstaged, thus at low powers (and hence low air mass flows), fuel isdelivered by the central pilot fuel injector 313 at a rich fuel-airratio (i.e. at an equivalence ratio greater than unity) for improvedflame stability. In the present example, as power and mass flowincreases, a staging point (SP) is reached at which fuel is delivered bysome or all of the main fuel injectors of the fuel nozzles,supplementing the fuel flow from the pilot fuel injectors. The main fuelinjectors 314 are configured to inject fuel at a lean fuel-air ratio(i.e. at an equivalence ratio less than unity). At this point, airflowis such that the equivalence ratio immediately downstream of the pilotfuel injectors is also fuel-lean. In the present example, at higherpower levels, fuel is injected by all main fuel injectors.

The staged combustion system is therefore characterised by a“pilot-only” region of operation and a “pilot-and-main” region ofoperation. The two regions are separated by the staging point. In“pilot-only” operation, all fuel flow provided within the combustor isprovided solely by pilot fuel injector(s). In “pilot-and-main”operation, fuel is provided within the combustor by the pilot fuelinjector(s) and by main fuel injector(s), or just the main fuelinjectors. The fuel delivery regulator 306 is therefore arranged todeliver fuel to the main fuel injectors, or both the main and pilot fuelinjectors, during pilot-and-main operation. The fuel delivery regulatormay be arranged to deliver fuel to both the pilot and main fuelinjectors during pilot-and-main operation rather than switching off thepilot fuel injectors in case of rapid emergency operation to keep thecombustor lit. During pilot-only operation the fuel delivery regulatorprovides fuel only to the pilot fuel injectors.

For a particular operating condition such as altitude or ambient airpressure, the staging point SP is typically defined by a threshold valueof an engine operating parameter representative of engine power setting,such as T30 (total air temperature at compressor exit) or calculated T40(total air temperature at combustor exit) or combustor Fuel Air Ratio(FAR). It will be appreciated that different threshold values mightapply for different operating conditions.

Those skilled in the art will be familiar with such operation of stagedcombustion systems in order to affect lean burn at high powers whilstalso observing flame stability limits at lower powers. Furthermore, theywill also be familiar with other staged combustion configurations, forexample those with separate pilot and main fuel injectors (as opposed toduplex or internally-staged), which may be arranged in parallel(radially separate, axially aligned) or series (axially separate,radially aligned). It will be understood that the principles disclosedherein may be applied to any staged combustion system comprising mainand pilot fuel injectors.

The balance of injection of fuel by the pilot fuel injectors 313 and themain fuel injectors 314 is controlled by the electronic enginecontroller 42, which provides control signals to the fuel deliveryregulator 306 indicative of the total fuel that must be injected in theform of a fuel flow rate (WE) and the ratio of pilot fuel injector fuelflow to main fuel injector fuel flow (staging ratio). The fuel deliveryregulator 306 is configured to utilise these control signals to deliverthe demanded fuel flow rate in accordance with the demanded overall fuelflow and pilot-main fuel flow ratio. In alternative examples, theelectronic engine controller 42 may instead be configured to providecontrol signals to the fuel delivery regulator 306 which are indicativeof the pilot fuel flow rate (W_(Fpilot)) and the main fuel flow rate(W_(Fmain)) It will of course be appreciated that the informationconveyed is equivalent.

Prior art systems may simply provide fuel from a single source to a fueldelivery regulator so that fuel having the same fuel characteristics(i.e. same fuel composition) is provided to both the pilot fuelinjectors and main fuel injectors at all points of operation of thecombustor. In the present application however, the fuel deliveryregulator 306 is configured to selectively provide fuel to the main andpilot manifold from two different fuel sources as will be describedfurther below.

Staged Combustion Using Fuel from More than One Source

In prior art staged combustion systems there is access to fuel from onlya single source, which is used in both the pilot-only and pilot-and-mainranges of operation. Fuel having the same characteristics is thereforesupplied to the combustor fuel injectors by prior art fuel deliveryregulators regardless of the fuel flow rate. The inventors havedetermined that it may be advantageous to provide fuel from differentfuel sources having different characteristics to the fuel deliveryregulator, and delivering fuel to the pilot and main fuel injectors suchthat fuel of a different characteristic is provided in different rangesof the combustor operation. Referring again to the staged combustionsystem shown in FIG. 5 , the fuel delivery regulator 306 is thereforearranged to deliver fuel to the pilot fuel injectors 313 during at leastpart of the pilot-only range of operation (i.e. mode of operation)having a different fuel characteristic from fuel delivered to one orboth of the pilot 313 and main fuel injectors 314 during at least partof the pilot-and-main range of operation. The different fuelcharacteristic may be provided by using fuel from the first fuel source302, fuel from the second fuel source 304, or a blend thereof.

One advantage of this more flexible supply of fuel to the combustor 16is that fuel which has favourable combustion properties but is limitedin supply can be used where those favourable properties will have agreater effect.

nvPM Emission from Staged Combustion

FIG. 6 illustrates a relationship between fuel flow rate WE and sootparticle emission-rate (shown as “non-volatile particulate matter (nvPM)number” and referred to as “nvPM#” herein) for a typical lean-burncombustor (e.g. a staged combustor system). Any reference herein to sootmay apply equally to other types of nvPM.

FIG. 6 shows a first region, to the left of the staging point SP (atlower fuel flow rate than that at the staging point), in which only thecombustor's pilot fuel injectors are operating, and in which nvPM# risesrapidly with increasing fuel flow rate WE. Operation in this firstregion will be referred to as operation in the “pilot-only” range ofoperation.

FIG. 6 shows a second region, to the right of the staging point SP (athigher fuel flow rate than that at the staging point), in which both thepilot fuel injectors and the main fuel injectors are operating, and inwhich nvPM# is much lower and rises only slowly (if at all) withincreasing WE. Operation in this second region will be referred to asoperation in the “pilot-and-main” range of operation. Although anon-zero value of nvPM# within the “pilot-and-main” range of operationis shown in FIG. 6 , it will be appreciated that in some lean-burncombustors nvPM# within the “pilot-and-main” range could besubstantially zero.

It is to be understood that the form of the curve shown in FIG. 6 isonly one example provided for purposes of illustration. Generally thenvPM# in pilot-only operation is substantially higher than that inpilot-and-main operation, and there is a sharp boundary between the tworanges of operation. Some examples set out below take additionaladvantage of the positive gradient of the curve shown within pilot-onlyoperation, but again the precise form of the curve shown in FIG. 6 isonly one example of the dependency between WE and nvPM#.

The position of the staging point SP may be chosen such that the fuelflow rate during most or all cruise conditions falls to the right ofstaging point SP i.e. in a region of pilot-and-main operationcorresponding to very low (or perhaps substantially zero) values ofnvPM#. Conversely, operation at many other points of a flight, such astaxi, approach, descent, may correspond to “pilot-only” operation inwhich nvPM (soot) emissions are comparatively high.

It is to be understood that the staging point SP may lie at differentvalues of WE for different flight conditions such as altitude. Forexample, at cruise where the density of air is low and thus the massflow rate of working fluid through the gas turbine core is also low, thestaging point SP may correspond to a substantially lower fuel flow ratethan it would on the ground where ambient air density is much higher.

The staging point SP applying at a particular flight condition may bedefined as an absolute value of fuel flow rate, or as a predeterminedpercentage of whatever maximum WE applies at the flight condition. Otherdefinitions of the staging point could also be used as would beunderstood by the skilled person.

The present applications refers to nvPM number (or nvPM#) and itsdependency upon WE, as illustrated in FIG. 6 . However, it will beappreciated that a corresponding relationship for nvPM mass and itsdependency on WE could also be contemplated, and that the apparatus andmethods set out herein could be adapted accordingly.

Control of nvPM Emissions

The level of nvPM emission by a gas turbine engine is dependent on thecharacteristics of the fuel being used. For example, some aircraft fuelsmay be characterised by a lower proportion of certain constituents thatare known to cause soot emissions compared to typical fossil kerosene,and so produce a lower amount of soot at the same combustor fuel flowrate.

While the emission of nvPM by a gas turbine engine could be reducedeffectively by using fuel associated with a low index of nvPMproduction, that may not always be possible. For example, theavailability of such fuels may not be sufficient for use throughout theduration of a flight. There may also be other technical, regulatory orcost constraints on using such low nvPM fuels in large fuel volumes orin large concentrations within a blended fuel.

The inventors have determined that by favouring using fuel associatedwith a relatively low level of nvPM emission (e.g. compared to fossilkerosene) during the pilot-only range of combustor operation compared toduring the pilot-and-main range of operation the nvPM emissions canstill be significantly reduced, while not requiring use of a largeamount of the low nvPM fuel throughout the combustor range of operation.

In the present example therefore, the first fuel held in the first fuelsource 302 is associated with a level of nvPM production which is lessthan that of the second fuel held in the second fuel source 304 (e.g.when being used in corresponding conditions). The fuel delivered to thepilot fuel injectors during at least part of the pilot-only range ofoperation is also associated with a level of nvPM production which isless than the fuel delivered to one or both of the pilot and main fuelinjectors during at least part of the pilot-and-main range of operation.As discussed below fuel may be provided to the pilot and main fuelinjectors 313, 314 exclusively from one of the two available fuelsources 302, 304, or as a blend of fuel from the first and second fuelsources 302, 304.

In some examples, the fuel system may be configured such that the mainfuel injectors 314 can be supplied from either the first fuel source 302or the second fuel source 304, and also such that the pilot fuelinjectors 313 can be supplied from either the first fuel source 302 orthe second fuel source 304. Even though some of the examples presentedbelow do not include such a flexible fuel system, there are safetyadvantages in ensuring that any fuel tank can supply fuel to any fuelinjector of any engine.

In some of the examples described herein, an individual fuel injector313, 314 is supplied either with the fuel only from the first fuelsource 302 or fuel from the second fuel source 304, i.e. the fuel systemswitches between the two fuel sources.

In other examples, the fuel system also comprises the necessaryequipment to perform blending of fuel from the two on board fuel sources(e.g. high soot index and low soot index fuels), the blending ratiobeing varied according to various decision making criteria in order toproduce a fuel composition whose characteristics are equal to those ofthe low soot producing fuel composition (100:0 blending ratio) or equalto the high soot producing fuel composition (0:100 blending ratio) orsomewhere in between (x:100−x blending ratio where 0<x<100).

Various examples may involve the ability to switch betweenpre-determined fuel compositions and/or to produce a blended fuelcomposition for only the pilot fuel injectors 313. The main fuelinjectors 314 may in such examples be supplied at all times with fuelfrom one of the fuel sources (e.g. the high nvPM associated fuel).However, for more flexibility, some examples may allow the main fuelinjectors 314 to be switched to the fuel from the other source (e.g. thelow nvPM fuel composition) and/or a blended fuel composition duringcertain abnormal periods of operation, for example in case of a loss ofhigh nvPM associated fuel due to for example a fuel leak.

In some examples, the fuel characteristic by which fuel from the firstfuel source 302 differs from fuel from the second fuel source 304 may bea percentage of sustainable aviation fuel (SAF) present in therespective fuel. The fuel delivered to the pilot fuel injectors 313during at least part of the pilot-only range of operation wouldsimilarly have a different percentage of SAF compared to the fueldelivered to one or both of the pilot fuel injectors 313 and main 314fuel injectors during at least part of the pilot-and-main range ofoperation.

Compared to fossil kerosene, SAF offers substantially lower emissions ofsoot, or more generally, nvPM. When SAF is used as part of a blendedfuel-composition with fossil kerosene, broadly speaking the higher thepercentage of SAF in the blend (and hence the lower the percentage offossil kerosene) the greater is the reduction in nvPM emissions, atleast within some ranges of SAF percentage. This effect is illustratedin FIG. 7 , which shows nvPM# emissions dependencies upon fuel flow rateWE for a default fuel composition (solid line) and for three furtherfuel compositions A, B and C (labelled) for a particular flightcondition defined by for example altitude and forward speed. The defaultfuel composition and the fuel compositions A, B and C are characterisedrespectively by progressively higher SAF percentages and correspondinglylower fossil kerosene percentages. The default fuel composition could be100% fossil kerosene, or it could be a blend comprising mostly fossilkerosene with a small percentage of SAF, such as may be available bydefault at some airports.

In FIG. 7 the reduction factor in nvPM# emissions relative to thedefault fuel composition achieved by each blended fuel composition isshown as being unchanging with WE during pilot-only operation and alsounchanging with WE during pilot-and-main operation, although thereduction factors in those two regions of operation are shown as beingdifferent from each other. It will be appreciated that otherdependencies upon WE may be observed in various implementations and arealso contemplated by the present application.

FIG. 8 shows one such alternative dependency upon WE, characterisedwithin pilot-only operation by an increase in the nvPM# reduction factorfor each blended fuel composition as WE is reduced from the stagingpoint SP towards a lower level of fuel flow. Other variations could bepossible; in particular the dependency upon WE of the nvPM# reductionfactor need not be the same for each blended fuel composition, and/orneed not be a monotonically increasing or monotonically decreasingfunction of WE. Although subsequent examples in the present applicationare based upon the form shown in FIG. 7 , it will be understood thatmore general forms are also contemplated.

The inventors have determined that when a gas turbine engine 10 isoperating in the pilot-and-main range of operation the soot emissionsare inherently low and the substitution of default fuel composition(e.g. fossil kerosene) with SAF (or a high-percentage SAF blend) orother low soot producing fuel will produce little further reduction insoot emissions. Conversely when the engine 10 is operating at a fuelflow rate which is below, but nonetheless close to, the staging pointSP, soot emissions can be substantially reduced by using a fuelcomposition which comprises a higher percentage of low soot producingfuel (e.g. SAF) and a lower percentage of fossil kerosene (or other highsoot producing fuel) relative to a default fuel composition. By usingfuel having a different characteristic in different ranges of thecombustor operation the percentage of SAF within the fuel compositionburned in pilot-only mode can be increased (e.g. maximised), whilereducing (e.g. minimising) the percentage of SAF within the fuelcomposition burned in pilot-and-main mode. This may allow the nvPMreducing effect of the available SAF to have a greater impact comparedto using a constant percentage composition of SAF throughout the fullrange of combustor operation.

In various other examples another characteristic or characteristics ofthe fuel may be varied alternatively to, or in addition to, thepercentage of SAF. Changes in other characteristics between the fuels ofthe first and second fuel sources may additionally or alternatively beassociated with different levels of the nvPM#. For example, the firstfuel source 302 may be characterised by fuel with a lower proportion ofcertain constituents which cause soot or other nvPM emissions comparedto that of the second fuel source 304. In some examples, the fuel of thefirst fuel source 302 may be characterised by a lower aromatichydrocarbon content, or more specifically lower naphthalene contentcompared to the second fuel source 304. Lower content of such sootproducing compounds may be associated with SAFs when compared to fossilkerosene. That may however not always be the case. Some SAFs may beassociated with a higher level of soot production compared to others(e.g. if they have a greater amount of a synthetic aromatic contentadded), or may be associated with a higher level of soot productioncompared to a fossil fuel such as fossil kerosene from which aromaticcontent has been removed in order to leave predominantly paraffiniccontent.

Various examples of delivering fuel to the main and pilot fuel injectors313, 314 from either of the first and second fuel sources 302, 304(exclusively, at least under normal running conditions) or a blendthereof at various points during the operating ranges of the combustorare described below.

FIGS. 9, 10 and 11

FIG. 9 illustrates an example in which the fuel delivery regulator 306is arranged to deliver fuel from the first fuel source 302 to the pilotfuel injectors 313 during operation in both the pilot-only and thepilot-and-main ranges of operation, and fuel from the second fuel source304 to the main fuel injectors 314 during operation in thepilot-and-main range of operation. In this example, the fuel deliveryregulator 306 comprises a pilot regulator 306 a in fluid communicationwith the first fuel source 302 via the first fuel pump 308 a. The fueldelivery regulator 306 further comprises a separate main regulator 306 bin fluid communication with the second fuel source 304 via the secondfuel pump 308 b. The pilot regulator 306 a is arranged to deliver fuelto the pilot manifold 309 and the pilot fuel injectors 313. The mainregulator 306 b is arranged to deliver fuel to the main manifold 310 andthe main fuel injectors 314. The fuel delivery regulator 306 thereforecomprises two separate flow paths via which fuel from each fuel source302, 304 is provided to the combustor 16. The rate of flow of fuelthrough each of the pilot regulator 306 a and main regulator 306 b maybe controlled independently from each other using control signalsreceived by the fuel delivery regulator 306 from the EEC 42. The pilotfuel injectors 313 are therefore at all times supplied with fuel fromthe first fuel source 302 and the main fuel injectors 314 are at alltimes supplied with fuel from the second fuel source 304. The fuel flowto the main fuel injectors 314 may be substantially zero in thepilot-only range of operation. This means that the fuel compositionpassing through an individual fuel injector does not change throughoutthe flight and is pre-determined prior to flight (at least during normalrunning conditions).

In the example of FIG. 9 the first fuel source 302 contains a fuel thatis associated with a low nvPM production, for example a fuel having arelatively high SAF content (e.g. a SAF rich fuel). The second fuelsource 304 contains a fuel associated with a high nvPM production, forexample a fuel having a relatively low SAF content (i.e. lower than thefirst fuel), e.g. a SAF poor fuel. The term ‘SAF-rich’ may be usedherein to indicate a fuel having a higher SAF content to a ‘SAF poor’fuel. The term ‘SAF-rich’ fuel may include fuel that is 100% SAF. The‘SAF poor’ fuel may include fuel that is 0% SAF, e.g. fossil kerosene.In some examples, the SAF-rich fuel may include up to 50% SAF, and theSAF-poor fuel substantially zero % SAF e.g. may be standard fossilkerosene fuel.

In the example of FIG. 9 , the SAF-rich fuel composition may bedetermined by the one or more of the following factors:

-   -   a) The amount of SAF available to, or allocated to, a proposed        flight;    -   b) The total fuel requirement for the pilot fuel injectors for        the entire flight (calculated according to methods known to the        skilled person); and    -   c) Any limits on the maximum allowable percentage of SAF, for        example certification limits, or for example technical limits        related to the specific aircraft and/or to the pilot fuel        injectors themselves, or the maximum percentage blend in which        SAF is available at the point of fuel loading.

The desired SAF percentage in the SAF-rich fuel composition may becalculated as 100% times factor a), divided by factor b), subject to amaximum allowable value which is the minimum of the various potentiallimits identified in factor c). An adjustment may be necessary to takeinto account the different volumetric energy densities of SAF and fossilkerosene, using methods known to the skilled person.

In the present example the SAF-poor fuel composition may have a defaultSAF percentage of zero (or the minimum possible given the default fuelsupply at the airport at which the aircraft is fuelled), but any SAFallocated to the proposed flight which was not incorporated into theSAF-rich fuel composition for the pilot fuel injectors 313 will be usedas part of the SAF-poor fuel composition for the main fuel injectors314. The resulting percentage of SAF in the SAF-poor fuel compositionwill be capped by any certification limits or by any technical limitsrelated to the specific aircraft and/or to the main fuel injectors, orthe maximum percentage blend in which SAF is available at the point offuel loading.

Operation of the combustor in FIG. 9 may not be possible for all flightsdue to the size of the available fuel tanks for SAF-rich fuelcomposition and for SAF-poor fuel composition not being able toaccommodate the fuel volume requirements for one or both of respectivelythe pilot fuel injectors 313 and main fuel injectors 314. This may bethe case for long range flights in which all fuel tanks need to be fullyfilled prior to departure and for which the respective fuel volumerequirements for the various fuel injector types may not correspondexactly to the source volumes.

FIG. 10 illustrates the dependency of nvPM number upon fuel flow rate WEfor the fuel regulator arrangement of FIG. 9 (dashed line) in comparisonwith the corresponding dependency for a default fuel composition such asfossil kerosene (solid line). In this illustration it is assumed thatthe SAF-rich fuel composition corresponds to fuel composition A fromFIG. 7 . As can be seen in FIG. 10 , an advantageous reduction in nvPMnumber is provided within the pilot-only range of operation, whereas inthe pilot-and-main range of operation there is little or no change innvPM number. In this example, a limited amount of available SAF hastherefore been more effectively targeted to a part of the range ofoperation (i.e. pilot-only) where it can bring the greatest advantage interms of nvPM reduction.

FIG. 11 illustrates a method 4000 of operating a gas turbine enginewhich may be performed using the staged combustor system of FIG. 9 . Themethod 4000 comprises regulating 4002 fuel delivery to the pilot andmain fuel injectors 313, 314 from the first fuel source 302 and thesecond fuel source 304. As discussed above, regulating the fuel deliverygenerally comprises delivering fuel to the pilot fuel injectors 313during at least part of the pilot-only range of operation having adifferent fuel characteristic from fuel delivered to one or both of thepilot fuel injectors 313 and main fuel injectors 314 during at leastpart of the pilot-and-main range of operation. In the example of FIG. 11, regulating 4002 the fuel delivery comprises delivering 4004 fuel fromthe first fuel source 302 to the pilot fuel injectors 313 duringoperation in both the pilot-only and the pilot-and-main ranges ofoperation, and fuel from the second fuel source 304 to the main fuelinjectors 314 during operation in the pilot-and-main range of operation.Any of the other features described above in connection with FIG. 9 maybe incorporated into the method of FIG. 11 , even though they are notrepeated here.

FIGS. 12, 13 and 14

In the example shown in FIGS. 9, 10 and 11 , the advantageous reductionin nvPM number (relative to the default fuel composition) observedparticularly within pilot-only operation (i.e. to the left of point SP)is made possible by the prioritisation of SAF to the pilot fuelinjectors, thus enabling a higher SAF percentage in the fuel compositionsupplied to those fuel injectors. However, during pilot-and-mainoperation (in which soot production is minimal even when running onfossil kerosene) nonetheless a SAF-rich fuel composition is still beingsupplied to the pilot fuel injectors. A yet more efficient use of SAFrich fuel may be obtained by providing further flexibility of fueldelivery to the combustor.

FIG. 12 illustrates an example in which the fuel delivery regulator 306is arranged to switch delivery of fuel to the pilot fuel injectors 313between the first fuel source 302 and the second fuel source 304. In thedescribed example the switching occurs at the boundary of the pilot-onlyrange of operation (e.g. at the staging point SP). In other examples,there may be multiple switching points including at the boundary and/orwithin the pilot-only range of operation. More generally therefore, thefuel delivery regulator 306 is arranged to switch delivery of fuel tothe pilot fuel injectors 313 between the first and second fuel source302, 304 at one or more operating points within, or at a boundary of,the pilot-only range of operation.

Referring to FIG. 12 , the pilot regulator 306 a comprises two separateindependently controllable regulators, a first regulator 315 a in fluidcommunication with the first fuel source 302, and a second regulator 315b in fluid communication with the second fuel source 304. The first andsecond regulators 315 a, 315 b of the pilot regulator 306 a arecontrollable by signals received from the EEC 42. Both of the first andsecond regulators 315 a, 315 b are in fluid communication with the pilotmanifold 309 so that they may control the delivery of fuel to the pilotfuel injectors 313.

The fuel delivery regulator 306 is arranged to switch delivery of thefuel to the pilot fuel injectors 313 between fuel from the first fuelsource 302 and fuel from the second fuel source 304 according to a modesignal indicative of a change in the range of operation of the stagedcombustion system. The mode signal may be obtained by the EEC 42 fromthe combustion system 64 (or combustor 16) and a corresponding controlsignal sent to the regulators 315 a, 315 b so that the switching mayoccur at the staging point between the pilot-only range of operation andthe pilot-and-main range of operation. The fuel delivery regulator 306is thus arranged to switch the pilot fuel injectors 313 between thefirst fuel and the second fuel (or vice versa) each time the stagingpoint SP is crossed. The EEC may be configured to receive from thecombustion system 64 (or combustor 16) a signal indicative of its modeof operation (pilot-only or pilot-and-main). Alternatively, the EEC 42may instruct the combustion system 64 to switch from one mode ofoperation to the other. In this example, the mode signal may already beavailable at the EEC 42, and may be used to switch fuel delivery by thefuel delivery regulator 306. Switching delivery of the fuel according toa mode signal indicative of a change in mode of the combustor 16 mayensure an acceptable level of synchronisation between the combustor'smode of operation and the pilot fuel injectors' fuel composition. Thismay help to ensure quick switching between fuel sources as the stagingpoint is crossed.

In the presently described example, the pilot regulator 306 a isarranged to switch between supplying fuel exclusively from the firstfuel source 302 and exclusively from the second fuel source 304. Thefirst and second fuel regulators 315 a, 315 b may therefore be arrangedto switch between operating where: i) the first is fully closed (so thatno fuel from the corresponding source is provided to the combustor 16),and the second is used to control the rate of flow from the other fuelsource; and ii) the second is fully closed (so that no fuel from thecorresponding source is provided to the combustor 16), and the first isused to control the rate of flow from the other fuel source. In otherexamples, any other suitable arrangement of pilot fuel regulator 306 amay be provided in order to allow switching between fuel sources.

The fuel delivery regulator 306 illustrated in FIG. 12 may be arrangedto deliver fuel such that when the engine 10 is operating in pilot-onlymode, the pilot fuel injectors 313 are supplied with a SAF-rich fuelcomposition, and when the engine is operating in pilot-and-main mode,both the pilot fuel injectors 313 and the main fuel injectors 314 aresupplied with a SAF-poor fuel composition.

In this example, the SAF-rich fuel composition will be determined by thefollowing factors:

-   -   a) The amount of SAF available to or allocated to a proposed        flight;    -   b) The total fuel requirement for the pilot fuel injectors        during pilot-only operation for the entire flight (calculated        according to methods known to the skilled person); and    -   c) Any limits on the maximum allowable percentage of SAF, for        example certification limits, or for example technical limits        related to the specific aircraft and/or to the pilot fuel        injectors themselves, or the maximum percentage blend in which        SAF is available at the point of fuel loading.

The desired SAF percentage in the SAF-rich fuel composition is thensimply 100% times factor a) divided by factor b), subject to a maximumallowable value which is the minimum of the various potential limitsidentified in factor c). An adjustment may be necessary to take intoaccount the different volumetric energy densities of SAF and fossilkerosene, using methods known to the skilled person.

The SAF-poor fuel composition may be determined using the same methodand constraints as for the example described in connection with FIGS. 9,10 and 11 , additionally constrained by any practical limits related tothe pilot fuel injectors 313 if required.

FIG. 13 illustrates the dependency of nvPM number upon fuel flow rate WEfor the fuel delivery regulator of FIG. 12 (dashed line) in comparisonwith the corresponding dependency for a default fuel composition such asfossil kerosene (solid line). In this illustration it is assumed thatthe SAF-rich fuel composition corresponds to fuel composition B fromFIG. 7 and that the SAF-poor fuel composition corresponds to the defaultfuel composition.

As can be seen in FIG. 13 , for a fixed amount SAF allocated to anindividual flight, the fuel delivery regulator 316 of FIG. 12 enablesthe SAF-rich fuel composition to have a higher percentage SAF contentcompared to the example of FIG. 9 , due to further restricting use ofthe SAF-rich fuel composition to pilot-only operation. As a result, SAFis more effectively prioritised to a region of operation in which theadvantageous reduction in nvPM is greater.

As with the example of FIGS. 9, 10 and 11 , fuel-tank capacities mayprevent operation according to the example of FIG. 12 for some flights,because it may be difficult to match the volumes of the various fueltanks to the required volumes of the SAF-rich and SAF-poor fuelcompositions.

FIG. 14 illustrates a method of operating a gas turbine engine 10 whichmay be performed using the staged combustor system of FIG. 12 . Methodsteps common to the method of FIG. 11 are labelled accordingly. In thisexample, regulating 4002 the fuel delivery comprises switching 4006delivery of the fuel to the pilot fuel injectors 313 between the firstfuel source 302 and the second fuel source 304 at the staging point SP.Any of the features described above in connection with FIG. 12 may beincorporated into the method of FIG. 14 , even though they are notrepeated here.

FIGS. 15 and 16

The fuel delivery regulator of FIG. 12 may in some examples be arrangedto further switch delivery of fuel to the pilot fuel injectors 313between the first fuel source 302 and the second fuel source 304 at athreshold point TP within the pilot-only range of operation. Fuel fromthe second fuel source 304 may be delivered to the pilot fuel injectors313 at fuel flow rates below the threshold point, fuel from the firstfuel source 302 delivered to the pilot fuel injectors 313 at fuel flowrates between the threshold and the boundary of the pilot-only range ofoperation (staging point), and fuel from the second fuel source 304 maybe delivered to the pilot fuel injectors 313 at fuel flow rates abovethe boundary. The main fuel injectors 314 may be supplied with fuel fromthe second fuel source 304 at all times.

Similarly to the previously described examples, the first fuel containedin the first fuel source 302 may be a low nvPM associated fuel such as aSAF-rich fuel, while the second fuel contained in the second fuel source304 may be associated with high nvPM such as a SAF-poor fuel. When thecombustor 16 is operating in pilot-only mode close to the staging pointSP (as determined by the position of the threshold) the pilot fuelinjectors 313 are supplied with the SAF-rich fuel composition. At allother times the pilot fuel injectors 313 are supplied with a SAF-poorfuel composition.

This example may provide a still more efficient use of SAF and may befurther advantageous compared to examples where switching occurs only atthe staging point SP in cases where the SAF percentage in the SAF-richfuel composition of the second example is limited by availability of SAFrather than by certification limits of the engine. The present examplein which two switching points are provided may also overcomedifficulties with potential mismatches between individual fuel-tankcapacities and the required volumes of SAF-rich and SAF-poor fuelcompositions.

The position of the switching threshold TP may be determined accordingto one or more different factors so that the switching occurs “close” tothe staging point SP. For example, the threshold may be:

-   -   a) a first threshold fuel flow rate beyond which the production        of nvPM by the gas turbine engine 10 passes a threshold amount        of the nvPM produced by the gas turbine engine during operation        in which the pilot fuel injectors 313 are delivered fuel having        the second fuel composition. For example, the threshold may be        defined as operation in pilot-only mode at a fuel flow rate        which, for a default fuel composition such as fossil kerosene,        would correspond to soot emissions exceeding a threshold. The        soot emissions or nvPM produced by the gas turbine engine may be        defined either as a number of soot particles emitted per unit        mass of fuel (i.e. a number emission index) or as a number of        soot particles emitted per unit time (i.e. also taking into        account the fuel flow rate) or as a number of soot particles        emitted per unit distance of flight (i.e. also taking into        account the speed of the aircraft).    -   b) a second threshold defined as a predefined threshold fuel        flow rate less than the fuel flow rate at the staging point SP        (as defined for the current flight conditions such as the        current altitude). The predefined threshold may be either a        percentage of the fuel flow rate at the staging point or an        absolute value of flow less than that at the staging point.

The first threshold and/or the second threshold may be defined withreference to the quantity of SAF available for a proposed flight,working on the assumption that as much as possible of the available SAFwill be incorporated into the SAF-rich fuel composition, subject topreviously-identified constraints, and taking into account the fuelvolume requirement for a proposed flight for the pilot fuel injectorswhen operating “close to point SP” according to a candidate value of thefirst threshold and/or the second threshold, the fuel volume requirementbeing determined using methods familiar to the skilled person.

The second threshold may be defined with reference to the fuel-tankvolume available on board the aircraft 1 for the SAF-rich fuelcomposition, again taking into account the fuel volume requirement for aproposed flight for the pilot fuel injectors when operating “close topoint SP”, the fuel volume requirement being determined using methodsfamiliar to the skilled person. For some flights, particularlylong-range flights for which all available fuel capacity must be used,the size of the available fuel tanks 53, 55 may limit the availableoptions concerning the second threshold, to ensure that capacity of thefuel tank used for the SAF-rich fuel composition is fully used. Thiswould place corresponding constraints on SAF percentage within theSAF-rich fuel composition

In cases where the first threshold and/or the second threshold have notbeen defined with reference to the available quantity of SAF, theSAF-rich fuel composition may be determined by one or more of followingfactors:

-   -   a) The amount of SAF available to or allocated to a proposed        flight;    -   b) The total fuel requirement for the pilot fuel injectors        during pilot-only operation close to the staging point for the        entire flight (calculated according to methods known to the        skilled person and taking account of relevant fuel-tank volumes        as described above); and    -   c) Any limits on the maximum allowable percentage of SAF, for        example certification limits, or for example practical limits        related to the specific aircraft and/or to the pilot fuel        injectors themselves, or the maximum percentage blend in which        SAF is available at the point of fuel loading.

The SAF percentage in the SAF-rich fuel composition is then simply 100%times factor a) divided by factor b), subject to a maximum allowablevalue which is the minimum of the various potential limits identified infactor c). An adjustment may be necessary to take into account thedifferent volumetric energy densities of SAF and fossil kerosene, usingmethods known to the skilled person.

The SAF-poor fuel composition may be determined using the same methodand constraints as for example described in connection with FIG. 12 .

FIG. 15 illustrates the dependency of nvPM number upon fuel flow rate WE(dashed line) for an example in which two switching points are provided,one at a threshold TP within the pilot-only range of operation and asecond TP2 at the staging point SP, in comparison with the correspondingdependency for a default fuel composition such as fossil kerosene (solidline). In this illustration it is assumed that the SAF-rich fuelcomposition corresponds to fuel composition C from FIG. 7 and that theSAF-poor fuel composition corresponds to the default fuel composition.The switching points TP, TP2 can be seen by the rapid changes in nvPMnumber at the corresponding fuel flow rate.

FIG. 16 illustrates a method 4000 of operating a gas turbine engine 10which may be performed using the staged combustor system of FIG. 12 inwhich two switching points TP, TP2 are provided. Method steps common tothe method of FIG. 14 are labelled accordingly. In this example, theregulating 4002 of the fuel delivery further comprises a step ofswitching 4008 delivery of fuel to the pilot fuel injectors between thefirst fuel source 302 and the second fuel source 304 at a thresholdpoint within the pilot-only range of operation in addition to theswitching 4006 at the staging point. Any of the features described abovein connection with FIG. 15 may be incorporated into the method of FIG.16 .

FIGS. 17, 18 and 19

In the previously described examples, the fuel supplied to the pilot andmain fuel injectors 313, 314 is limited to the characteristics of thetwo pre-defined fuels (the first and second fuels) contained within thefirst and second fuel sources 302, 304. The inventors have determinedthat further advantages can be obtained by providing fuel containing ablend of fuel from the first and second fuel sources 302, 304 to thecombustor 16 to give greater flexibility of the fuel compositionprovided to the fuel injectors 313, 314.

FIG. 17 illustrates an example in which the fuel delivery regulator 306comprises a fuel blender 318. The fuel blender 318 is arranged toreceive a supply of fuel from both the first and second fuel sources302, 304 and output fuel from the first fuel source 302, fuel from thesecond fuel source 304, or a blend thereof (e.g. a blending ratio whichcan vary between 100% of the first fuel and 0% of the second fuel, to 0%of the first fuel and 100% of the second fuel and any ratio in between).The fuel blender 318 is in fluid communication with the pilot manifold309, and is arranged to deliver fuel to the pilot fuel injectors 313.The fuel delivery regulator 306 comprises a main regulator 306 bconnected to the second fuel source 304 and arranged to supply the mainmanifold 310 and main fuel injectors 314 similarly to other examples. Inother examples, the blender may be arranged to supply both the main andpilot fuel injectors 313, 314. By using the fuel blender 318 a blend offuel may be delivered to the pilot fuel injectors 313 during at leastpart of the pilot-only range operation. For other parts of thepilot-only operation, and during pilot-and-main operation, the pilotfuel injectors 313 may be provided with fuel from only one of the fuelsources by the fuel blender 313. A blend of fuel from both sources mayalso be provided during all of the pilot-only range of operation and/orduring the pilot-and-main range of operation.

Similarly to the previously described examples, the first fuel containedin the first fuel source 302 may be a low nvPM associated fuel such as aSAF-rich fuel, while the second fuel contained in the second fuel source304 may be associated with high nvPM emission such as a SAF-poor fuel.When the combustor 16 is operating in pilot-only mode a blend of fuelmay be provided to the pilot fuel injectors 313 so that the blended fuelprovided is associated with a lower nvPM compared to fuel provided tothe main and/or pilot fuel injectors 313, 314 during pilot-and-mainoperation where nvPM is inherently lower. This may make yet moreefficient use of low nvPM producing fuel. The fuel supplied to the pilotfuel injectors 313 during at least part of the pilot-only operation maytherefore contain more of the first fuel compared to the fuel providedto the main fuel injectors 314 during pilot-and-main operation. In theexample of FIG. 17 , the fuel delivery regulator 306 allows fuels fromthe first and second sources 302, 304 to be mixed by the fuel blender318 at a desired blend ratio and supplied to the pilot manifold 309.This is in contrast to the example of FIGS. 12, 13 and 14 , where ablend of fuel formed by mixing an amount of fuel from the first source302 and an amount of fuel from the second source 304 cannot be provided.

A number of advantages are associated with providing blended fuel inthis way. For example, a hard boundary or switching point (e.g.threshold point TP described above) within the pilot-only region, belowwhich SAF-poor fuel composition is supplied to the pilot fuel injectors313 and above which SAF-rich fuel composition is supplied to the pilotfuel injectors 313, may be avoided. This may reduce the risk of a suddenincrease in soot emissions as fuel flow is reduced, which couldotherwise result in unnecessarily high soot emissions during, forexample, approach and/or final approach flight phases.

Blending fuel may also be further advantageous because a more nvPMreducing blend of fuel can be used close to the staging point SP (i.e.at fuel flow rates just below the staging point) where nvPM number is ata maximum. This can provide greater overall control of nvPM emission,and more effectively make use of low nvPM fuels.

The percentage of SAF within the SAF-rich first fuel may be as high aspossible, subject to any limits on the maximum allowable percentage ofSAF, for example certification limits, or for example technical limitsrelated to the specific aircraft and/or to the pilot fuel injectorsthemselves, or the maximum percentage blend in which SAF is available atthe point of fuel loading.

The percentage of SAF within the SAF-rich first fuel may also beconstrained by the required filling-factor of the fuel tank(s) used forthe SAF-rich fuel composition, in conjunction with the quantity of SAFallocated to the proposed flight. For example, on long range flightsrequiring complete filling of all fuel tanks 53, 55, the volume ofSAF-rich fuel composition may not be less than the capacity of thesmallest individual fuel tank that could comprise the first fuel source.

The SAF-poor fuel composition, i.e. the second fuel, may be determinedusing the same method and constraints as for the previously describedexamples.

In some examples, the fuel delivery regulator 306 may be arranged toprovide a constant ratio blend of fuel from the first fuel source 302and fuel from the second fuel source 304. This may allow a fuel to beprovided to the combustor having characteristics different from thefuels available to supply the aircraft 1. This may provide furtherflexibility and improved nvPM control. For example, the blend ratio maybe determined and fixed for a particular flight once the amount of thefuel in the first and second fuel sources 302, 304 allocated to theflight is known.

In other examples, the fuel blender 318 is arranged to deliver a blendof fuel to the pilot fuel injectors 313 having a varying blend ratio offuel from the first fuel source 302 and fuel from the second fuel source304. The blend ratio may be varied within the pilot-only range ofoperation according to the fuel flow rate, or according to the fuel flowrate divided by the fuel flow rate at the staging point SP. In someexamples, fuel blend may be varied such that the proportion of fuel fromthe first fuel source 302 compared to that from the second fuel source304 is reduced with decreasing fuel flow rate within the pilot-onlyrange of operation. This may allow the amount of low nvPM producing fuel(e.g. SAF) to be a reduced as the flow rate reduces. In other examples,the opposite dependencies of first fuel content with fuel flow rate maybe provided. For example, in some cases, the percentage reduction innvPM due to the use of SAF may be greater at low power settings (e.g.low fuel flow) than at higher power settings (e.g. high fuel flow).

The fuel delivery regulator 306 of the example of FIG. 17 may bearranged to:

-   -   a) Deliver fuel from the second fuel source 304, e.g. high nvPM        producing fuel such as SAF-poor fuel for both the pilot fuel        injectors 313 and main fuel injectors 314 at fuel flow rates        above the staging point;    -   b) Deliver fuel to the pilot fuel injectors 313 from the first        fuel source 302 e.g. low nvPM producing fuel such as SAF-rich        fuel at and/or immediately below the fuel flow rate at the        staging point SP; and    -   c) Deliver fuel to the pilot fuel injectors 313 having a blend        comprising progressively less of the first fuel from the first        fuel source 302 and correspondingly more of the second fuel from        the second fuel source 304 as overall fuel flow rate is reduced        below the staging point.

The ratio of the first fuel and second fuel being supplied to the pilotfuel injectors 313 during pilot-only operation may be varied accordingto a fuel blending schedule. The fuel blending schedule may bedetermined by the EEC 42 and used to send control signals to the fueldelivery regulator 306 to control the blend ratio.

In some examples, the dependence of the proportion of fuel from thefirst fuel source 302 compared to that from the second fuel source 304on fuel flow rate may be determined according to a desired resultantlevel of nvPM at a particular fuel flow rate. For example, the blendratio may be determined such that the nvPM number for any specific fuelflow rate does not exceed a predetermined threshold. For a given flightcondition (such as altitude and forward speed), a lookup table may beused to determine the blend ratio (e.g. SAF percentage) necessary toachieve a particular level of nvPM number at a particular WE. Givenknowledge of the characteristic values of the first and second fuels(i.e. SAF percentage within each of the two predetermined fuelcompositions, SAF-rich and SAF-poor) the proportion of SAF-rich fuelcomposition required to be delivered to the pilot fuel injectors can bedetermined. The proportion of SAF-poor fuel blended with the SAF-richfuel to produce an instantaneous fuel composition to be supplied to thepilot fuel injectors 313 can then also be determined in order to keepnvPM production within a threshold limit throughout the pilot-onlyoperation. The determination of the proportion of each fuel making upthe blend may be determined by the EEC 42 based on information obtainedfrom the lookup table, information on the characteristics of the fuelcontained within the first and second fuel sources and the current fuelflow rate. Once a blend ratio is determined by the EEC 42 the blender318 may be controlled accordingly by control signals sent from the EEC42 to the fuel delivery regulator 306. In some examples, the EEC 42 maycalculate the blend ratio in real time in response to changes in currentflight condition or current atmospheric conditions.

In another example, the fuel blending schedule (taking account of thevariation with WE of the nvPM#(or nvPM mass) reduction factor for agiven fuel composition relative to that for the default or SAF-poorcomposition) may be determined so as to minimise (or keep within apredefined threshold) the total number (or mass) of emitted nvPM duringa period of operation of the gas turbine engine 10, such as a landingand take-off (LTO) cycle.

In advance of a flight, using knowledge of the desired dependency ofnvPM number upon WE and flight conditions, the characteristics of eachof the first and second fuels, and knowledge of how much fuel will beused at each value of WE at each flight condition, the total quantity ofeach of the first and second fuels that are required for a proposedflight can be determined as described later. The aircraft can thereforebe loaded with suitable amount of fuel before the flight.

FIG. 18 illustrates an example of the dependency of nvPM number uponfuel flow rate WE for the example shown in FIG. 17 (dashed line) incomparison with the corresponding dependency for a default fuelcomposition such as fossil kerosene (solid line). In this example, thefirst fuel is a SAF-rich fuel composition which corresponds to fuelcomposition C from FIG. 7 and the second fuel is a SAF-poor fuelcomposition that corresponds to the default fuel composition in FIG. 7 .

In the example of FIG. 18 , the characteristic of the fuel supplied tothe pilot fuel injectors 313 during pilot-only operation is determinedsuch that nvPM number does not exceed a predetermined threshold which inthe example shown corresponds to the nvPM number of the SAF-rich fuelcomposition at the staging point SP. In this example there is a regionof operation at low values of WE in which the pilot fuel injectors 313are supplied with the SAF-poor fuel composition. As WE increases andnvPM number rises, at some point it is necessary to start blending insome SAF-rich fuel composition in order to prevent nvPM number fromrising above the predetermined threshold. As WE rises further still, theproportion of SAF-rich fuel composition within the blend continues torise until, at the staging point SP, it reaches 100%. At still highervalues of WE, corresponding to pilot-and-main operation, the pilot fuelinjectors 313 are once again supplied with SAF-poor fuel composition.

Although FIG. 18 shows a capped nvPM number dependency upon WE, it willbe appreciated that through suitable determination of blending ratiodependency upon WE throughout the region of pilot-only operation, it ispossible to achieve any desired dependency of nvPM number upon WE,subject to an upper limit corresponding to the dependency of the secondfuel (i.e. the high nvPM associated fuel, such as the SAF-poor fuelcomposition) and a lower limit corresponding to the dependency of thefirst fuel (i.e. the low nvPM associated fuel, such as the SAF-rich fuelcomposition). If the SAF-rich fuel composition corresponds to fuelcomposition C from 7, the addressable region is shown by the hatchedarea in FIG. 19 .

Within the addressable region, any single-valued function of nvPM numberversus WE can in principle be achieved by a suitable blending schedulein dependency upon WE within pilot-only operation. FIG. 20 shows onefurther example of how the nvPM number may depend on fuel flow rate.

By providing a blend of fuel to the pilot fuel injectors in this waywithin pilot-only operation (or at least within one or more regionswithin pilot-only operation) the magnitude of the rate of change of nvPMnumber with WE can be made substantially less than that for the defaultfuel composition such as fossil kerosene, and also less than that forother examples in which switching between fuel sources 302, 304 occurs.As a result, within pilot-only operation, using the example of FIG. 17 ,WE could be varied for reasons other than soot emissions without causingvery substantial changes in soot emissions. For example, inUS2022042465, the fuel flow rates of individual engines are variedduring final approach in order to limit handling bleed noise. If weassume that the thrust requirement during final approach corresponds toa region of pilot-only operation, then the examples herein in which fuelis blended by the fuel blender 318 of the present invention maysubstantially reduce the change in soot emissions that would otherwiseaccompany such noise-motivated fuel flow changes.

The examples shown in FIGS. 13 and 15 can be thought of as special casesof the blended fuel example in which the SAF-rich fuel composition isused not only at the staging point SP but also throughout a regionextending some way to the left of point the staging point (i.e. at lowerfuel flow rates). In the example of FIG. 13 that region extends all theway to the left-hand side of the chart. In the example of FIG. 15 thereis a switching point at which the fuel composition is switched sharplyto the SAF-poor fuel composition within the pilot-only range ofoperation. It will be appreciated that switching from one fuel type tothe other is equivalent to changing the blending ratio from 0:100 to100:0, or vice versa.

FIG. 21 illustrates a method 4000 of operating a gas turbine engine 10which may be performed using the system of FIG. 17 . Steps common toother methods previously described are labelled accordingly. In thisexample, regulating 4002 the fuel delivery comprises: blending 4010 asupply of fuel from both the first and second fuel sources 302, 304 toform a blended fuel consisting of fuel from the first fuel source 302,fuel from the second fuel source 304, or a blend thereof; and delivering4012 the blended fuel to the pilot fuel injectors 313. Any of thefeatures described above in connection with FIG. 17 may be incorporatedinto the method of FIG. 21 , even though they are not repeated here.

nvPM Cost Function

In the examples described above control of the fuel characteristicsprovided to the fuel injectors is based on a desired form of thedependency of nvPM number upon WE from which the fuel characteristic(s)to be used at each value of WE are determined to enable that desiredform to be achieved. The fuel delivery regulator in these examples maytherefore be controlled to minimise nvPM production of the engine. ThenvPM production may be in terms of the nvPM content of the engineexhaust e.g. mass or number of nvPM particles produced.

In order to provide yet further improved control of nvPM emission,further factors may be taken into account because the inventors haveidentified that one emitted soot particle is not equally asdisadvantageous as another emitted soot particle. In some examplestherefore, the amount of cost or harm caused by each soot particleemitted can be taken into account. For example, soot particles emittedclose to the ground may be considered to have a higher human healthimpact than soot particles emitted several thousand feet above theground (indeed soot emissions less than 3000 ft above runway altitudeare regulated, whereas soot emissions at higher altitudes are not).

The fuel regulator of any of the examples described herein may bearranged to deliver fuel to the pilot fuel injectors in order to control(e.g. optimise or reduce) the nvPM impact of the engine exhaust. Thefuel delivery regulator 306 may therefore be arranged to deliver fuel tothe pilot fuel injectors to minimise a cost function dependent on one ormore nvPM impact parameters. The nvPM impact parameters may relate tothe cost or harm of nvPM (e.g. soot) emission of a particular type or ina certain situation. This may allow the gas turbine engine 10 to beoperated in such a way as to reduce nvPM emissions that would otherwisecause the most harm compared to those that have less an effect on theenvironment and/or human health.

The one or more nvPM impact parameters on which the cost function isbased may include any one or more of:

-   -   i) Height above ground level at which the nvPM production takes        place;    -   ii) Position (e.g. location e.g. longitude and latitude) of the        nvPM production. The cost of nvPM emission may be defined as a        function of 3D position of soot emissions (e.g. altitude,        longitude, latitude), and may take into account proximity to        population centres or other important locations;    -   iii) Weather/atmospheric conditions at a location of the nvPM        production. For example the cost function may take into account        prevailing winds, weather patterns and atmospheric behaviour        such as removal processes in order to assess the future location        of emitted soot particles relative to population centres or        other important locations;    -   iv) Climate impacts associated with location of the nvPM        production. For example, the climate impacts of the deposition        of soot in certain locations may be taken into account. Such        locations may include otherwise high-albedo surfaces, e.g. ice;    -   v) Mass/size of the individual nvPM particles produced. The cost        function may be defined so as to prioritise reductions of nvPM        number and/or mass in one or more specific particle size range,        relative to reductions of nvPM number and/or mass in one or more        further size range. The cost function may, for example, comprise        a weighted sum over the number (or mass) of particles in        different size ranges, the weighting defined so as to reflect        the notion that some size ranges might be more harmful to        health, or result in a more disadvantageous environmental        outcome, than others (and thus carry more importance within the        cost function), but without completely ignoring the other size        ranges;    -   vi) Potential contrail production and/or contrail        characteristics. For example, the cost function could take into        account the likelihood of certain nvPM emission to cause        production of a contrail by the aircraft, or to influence the        properties or characteristics of a contrail produced by the        aircraft. This may specifically apply to contrails formed at the        top of the aircraft descent phase where operation of the        combustor would likely be substantially below the staging point        and the use of low nvPM fuel can have a greater effect;    -   vii) local air quality (LAQ) impact of nvPM production; and/or    -   viii) Amount of nvPM produced. The cost function may take into        account the amount of nvPM emission produced by the engine e.g.        in terms of total mass and/or number of nvPM particles in a        given time, or an emissions index of the mass (or number) of a        nvPM per unit mass of fuel consumed.

Any number of the above impact parameters may be defined in order todefine a cost function that is to be minimised and the fuel deliveryregulator 306 controlled to provide fuel of the required characteristicsto the combustor 16. In some examples, the nvPM production cost functioncould be incorporated into a wider cost function comprising other costs.

The control of the fuel delivery regulator 306 of any example describedherein can be based on the cost function. For example, the on-boardblending of the first and second fuel (SAF-rich and SAF-poor) may beused to achieve a desired nvPM number at each operating condition withinpilot-only operation. The fuel characteristics supplied to the pilotfuel injectors 313 within pilot-only operation (and hence thecorresponding blending ratio of the quantity of the SAF-poor compositionto the quantity of the SAF-rich fuel composition) may no longer be asimple function of WE (indeed it may not even be a single-valuedfunction of WE) but may vary according to other parameters so as tominimise an overall cost function.

Switching Between Pilot-Only and Pilot-and-Main Operation During Cruise

In known staged combustion systems, the staging point SP is typicallychosen such that cruise operation takes place within the pilot-and-mainregion of operation. Operation at low-power settings, such as taxi,descent, and approach, often take place within the pilot-only region ofoperation.

The inventors have determined that it is further advantageous to operatea staged combustion system so that it is in pilot-only mode during atleast some of its cruise operation, while also selectively providingfuel from two different sources to the combustor during the cruiseoperation.

In another example of the present application, the staged combustionsystem 64 illustrated in FIG. 5 is additionally or alternativelyarranged to switch between the pilot-only range of operation and thepilot-and-main range of operation at a staging point selected so that itcorresponds to a steady state cruise mode of operation of the engine 10.The staging point in this example is arranged such that it defines aboundary between a first engine cruise operation range and a secondengine cruise operation range. In other words, the staging point isselected to occur at an engine power setting (or other engine operatingparameter indicative of the engine power setting) that is above theminimum engine power setting at which the engine operates in a steadystate cruise. This defines a range of relatively lower cruise enginepower conditions, in which the staged combustion system operates in thepilot-only mode of operation, compared to pilot-and-main operation atrelatively higher cruise engine power settings. This effectively movesthe staging point to a higher power setting such that the combustionsystem can still operate in pilot-only operation at higher powers thanthe minimum cruise power setting. The pilot-only and pilot-and-mainranges are therefore redefined such that the staged combustion system 64switches between modes of operation at a different WE i.e. the boundaryseparating the ranges is changed compared to prior art systems.

In this example, the fuel delivery regulator 306 is arranged to deliverfuel to the pilot fuel injectors 313 during at least part or preferablyall of the first cruise operation range having a different fuelcharacteristic from fuel delivered to one or both of the pilot fuelinjectors 313 and main fuel injectors 314 during the second cruiseoperation range. More specifically, during the first cruise operationrange, the fuel regulator 306 supplies fuel to the pilot fuel injectorsfrom the available fuel sources that is different from that supplied tothe fuel injectors during the second cruise operation range. This may befuel supplied to both main and pilot fuel injectors duringmain-and-pilot operation.

The inventors have determined that by setting the staging point so thatlower power cruise operation may take place in pilot-only mode certainengine emissions may be reduced and combustion efficiency improved. Whencombined with selectively using fuels having different characteristics,the inventors have determined that disadvantageous effects on emissionsthat would otherwise result in moving the staging point can bemitigated. This therefore provides an overall improvement in combustionefficiency and reduced emissions by a combination of these factors.

Emissions of pollutants may be characterised by an emission index (EI),detailing the mass (or number) of a particular pollutant per unit massof fuel consumed. The inventors have observed that in pilot-onlyoperation, at very low engine powers, the mass emission index of carbonmonoxide (CO) and of unburned hydrocarbons (HC) can be relatively high.Not only does this represent a release of pollutants to the atmosphere,it also reduces fuel efficiency due to incomplete combustion of fueli.e. not all fuel is fully burned. This may mean mission fuel burn isincreased and/or payload-range capability of the aircraft is reduced. Asengine power increases (still within pilot-only operation), the EI(CO)and EI(HC) both decrease and thereafter remain low. However, upon movingto still higher engine power settings and making the transition topilot-and-main operation, EI(CO) and EI(HC) again become high beforesubsiding once again at still higher engine power settings. This isillustrated FIG. 22 , which shows the dependence of EI(CO) and EI(HC) onengine power setting, where the vertical dashed line represents thestaging point SP.

During the first cruise operation range, the engine operates atrelatively low power settings, such that were the staging point selectedto cause the combustion system to operate in pilot-and-main mode, the COand HC emission would be relatively high. This is because even if thestaging point were chosen such that cruise operation takes place inpilot-and-main mode i.e. above the staging point SP, some parts ofcruise operation may lie fairly close to the staging point (e.g. justabove it). Disadvantageously this would mean that HC and CO emissionsmay be high during such operation, and combustion efficiency may bedisadvantageously and materially reduced as a result (resulting inhigher fuel consumption and/or reduced aircraft payload-rangecapability).

By setting the staging point so that low power cruise operation takesplace in pilot-only mode, this disadvantageous emission of CO and HC canbe reduced or avoided. This is illustrated in FIG. 23 , which shows theeffect of setting the staging point SP′ to define a first cruiseoperation range 320 a and a second cruise operation range 320 b. As canbe seen in FIG. 23 , the first cruise operation range 320 a correspondsto low power cruise, that is below the staging point SP′, with thesecond cruise operation range 320 b corresponding to higher power cruiseoperation above the staging point SP′. Within the first cruise operationrange 320 a, the staged combustion system 64 is configured to operate inpilot-only mode, thus providing low CO and HC emissions. As a result ofmoving the staging point to a higher power setting (e.g. from the thinto the thick dashed lines in FIG. 23 marked respectively SP and SP′), COand HC emissions remain low over a wider range of engine power settings(e.g. the thick solid line versus the thin dotted line in the dependenceof EI(CO) and EI(HC) on power setting).

The inventors have further observed that the emission index of nvPMrises rapidly with engine power in pilot-only mode but is typicallyuniformly low in pilot-and-main mode. For nvPM, the emissions index maybe described with reference to the mass of nvPM or to the number of nvPMparticles (per unit mass of fuel). An example of the dependence of nvPMon engine power setting is illustrated in FIG. 24 .

As a result of effectively moving the staging point to a higher powersetting, default EI(nvPM) is disadvantageously increased. As can beinferred from FIG. 24 , operation in pilot-only mode at higher powerswould lead to increased nvPM production as there would be less of theengine power range occurring in pilot-and-main operation where nvPMproduction is low, and the nvPM increases rapidly at higher powers inpilot-only mode of operation.

The inventors have determined that this otherwise detrimental increasein nvPM can be mitigated by the selective use of a fuel of a differentfuel characteristic during at least the part of the pilot-only cruiserange of operation at which high nvPM would otherwise be produced. Forexample, the fuel delivered to the pilot fuel injectors 313 during thefirst engine cruise operation range 320 a may be selected to be fuelassociated with a level of nvPM production which is less than that ofthe fuel delivered to one or both of the pilot and main fuel injectors313, 314 during at least part of the second engine cruise operationrange 320 b. This can be achieved by the fuel contained within the firstfuel source 302 being associated with low nvPM production compared tothe second fuel source 304 (at corresponding combustion conditionconditions).

The effect of selectively using fuel of a different characteristicduring the first engine cruise operation range 320 a is illustrated inFIG. 25 . The thick solid line in the EI(nvPM) dependence on power canbe compared to the thick dotted line in the first cruise operation range320 a that is produced by using a lower nvPM producing fuel.

In some examples, the fuel characteristic by which fuel from the firstfuel source 302 differs from fuel from the second fuel source 304 may bethe percentage of sustainable aviation fuel (SAF) present in therespective fuel as discussed above. The fuel delivered to the pilot fuelinjectors 313 during at least part of the pilot-only cruise range ofoperation would similarly have a different percentage of SAF compared tothe fuel delivered to one or both of the pilot fuel injectors 313 andmain fuel injectors 314 during at least part of the pilot-and-main rangeof operation. As discussed above, compared to fossil kerosene, SAFoffers substantially lower nvPM, and can therefore be used to mitigatethe increase in nvPM that would otherwise result from the additionalpilot-only cruise operation. For example, the fuel provided to the pilotfuel injectors during the first cruise operation range 320 a may have agreater proportion of SAF compared to that provided to the fuelinjectors during the second cruise operation range 320 b. This allows alimited available amount of SAF (or other low nvPM fuel) to be usedselectively to reduce overall nvPM, CO and HC emissions.

This may however not always be the case. In some examples, fossilkerosene may be treated to remove aromatic components, particularlynaphthalenes, in order to produce a largely paraffinic fuel of fossilorigin which would be a low nvPM producing fuel. Other fuelcharacteristics may therefore be associated with low nvPM, such as thepercentage of aromatic content or naphthalene content.

The inventors have therefore found that through a combination ofselectively using two fuel types and operating in pilot-only mode at lowpower cruise the advantageous reduction in CO and HC emissions isaccompanied by a smaller disadvantageous increase in nvPM than would bethe case without use of a SAF-rich fuel composition (or other low nvPMassociated fuel) in the relevant range of engine power settings.

The selective pilot-only cruise operation and selective fuel use of thepresent application is advantageous over other known methods to reduceoverall nvPM, HC and CO emission. For example, alternative solutionsinclude having a “pilot-and-half-main” region of operation which liesbetween the pilot-only region and the pilot-and-main region. When theengine power setting corresponds to the pilot-and-half-main region,instead of igniting all of the main fuel burners, only a proportion(e.g. half) of them are lit, with for example, an alternateon-off-on-off distribution around the combustor annulus (otherarrangements could be contemplated such as lighting main burners on onehalf of the annulus and not on the other half). However, this gives riseto uneven combustion properties around the combustor annulus and mayalso be disadvantageous for turbine operation and turbine life. Anotherapproach is to have a staged pilot system in which the “pilot-only”region of operation is divided into sub-regions, in which progressivelygreater numbers of pilot burners are switched on at progressively higherengine power settings within the overall “pilot-only” region. Such aconfiguration disadvantageously adds weight and complexity which can beavoided by the methods of the present application.

In one example, the first cruise operation range corresponds tooperation of the aircraft in a later part of a cruise segment of aflight, and the second operation range corresponds to operation of theaircraft in a relatively earlier part of the cruise segment. Forexample, during an aircraft flight cycle the cruise operation may bedivided into one or more cruise segments. These may correspond to steadystate cruise operation at different altitudes. Towards the end of acruise segment, the engine power setting required to maintain steadystate cruise at the specified Mach number and altitude will reduce asthe aircraft burns fuel and reduces in weight. As less thrust istherefore required at a later part of the cruise segment, the enginepower setting is reduced. The combustion system of the presentapplication may therefore be arranged to switch to pilot-only operationat low engine powers towards the end of a cruise segment, rather thanremain in pilot-and-main operation throughout the cruise segment (orsegments if there are more than one) of the flight. This helps to reduceHC and CO production that might otherwise occur towards the end of thecruise segment.

In another example, the first cruise operation range 320 a correspondsto steady state subsonic cruise operation of the engine and the secondcruise operation range 320 b corresponds to steady state supersoniccruise operation of the engine. In this example, the gas turbine engine10 is arranged to provide both subsonic and supersonic cruise operationof the aircraft to which it is mounted. The staging point may bedetermined such that the pilot-only cruise operation corresponds torelatively low engine power subsonic operation, whereas thepilot-and-main cruise operation corresponds to higher engine powersupersonic operation.

For a supersonic aircraft, supersonic cruise is likely to be possibleover oceans, but over land cruise may be constrained to be subsonic forcompliance with noise regulations. In such an example, supersonic cruisemay correspond to a high engine power setting, well above the stagingpoint. Subsonic cruise may however in this example correspond to muchlower engine power settings that might sit close to a default or priorart staging point, resulting in low combustion efficiency and highemissions of CO and HC during subsonic cruise. A supersonic aircraft mayconsume a material proportion of its fuel in subsonic cruise, and socombustion efficiency at that operating condition is of high importance.By operating in pilot-only mode during subsonic cruise, emissions of COand HC can be reduced, while fuel of a different characteristic can besupplied to minimise any increase in nvPM production. The steady statesupersonic cruise operation may occur before the steady state subsoniccruise operation, or vice versa. In some examples, the steady statesubsonic cruise operation may be subsonic cruise operation over land,while the steady state supersonic cruise operation may be supersoniccruise operation over water (e.g. over the sea).

In order to supply fuel to the pilot fuel injectors 313 during the firstcruise operation that is different from that supplied to the combustorduring the second cruise operation the fuel delivery regulator 306 isarranged to supply fuel selectively from the first and second fuelsources 302, 304 as described above (e.g. fuel from only the first fuelsource 302, fuel from only the second fuel source 304, or a mixturethereof). In one example, the fuel delivery regulator 306 may comprise afuel blender 318 as shown in FIG. 17 . Similarly to as described above,the fuel blender is arranged to receive a supply of fuel from both thefirst and second fuel sources 302, 304 and output a fuel consisting offuel from the first fuel source, fuel from the second fuel source, or ablend thereof. This blended fuel is supplied to the pilot fuel injectors313, with fuel from a single fuel source (the second fuel source 304 inthe example above) being supplied to the main fuel injectors 314. Inother examples, the blender may be arranged to supply both the main andpilot fuel injectors with an appropriate blend of fuel. In yet otherexamples, the fuel delivery regulator 306 of any of the examplesdescribed herein may be used to supply fuel in the first and secondcruise operation ranges (for example, those using separate regulatorsfor the first and second fuel sources or a pilot regulator arranged toswitch between fuel sources).

As discussed above in connection with the example shown in FIG. 17 , thedependence of the proportion of fuel from the first fuel source 302compared to that from the second fuel source 304 on fuel flow rate maybe determined according to a desired resultant level of nvPM at aparticular fuel flow rate. For example, the proportion of fuel deliveredfrom the first fuel source 304 compared to that from the second fuelsource 304 may be determined according to a desired resultant level ofnvPM at a particular fuel flow rate within the first engine cruiseoperation range. The amount of fuel supplied from each fuel source maybe determined such that the nvPM does not exceed a predeterminedthreshold or such that the nvPM production is minimised over a period ofoperation of the gas turbine engine as described above. In any of theexamples described herein this may be achieved by suitable control ofthe blender 318, or any of the other fuel delivery regulators 306described herein.

In some examples, the proportion of fuel delivered from the first fuelsource 302 compared to that delivered from the second fuel source 304during the first cruise operation range may be determined at leastpartly according to any one or more of:

-   -   a) the amount of fuel having the first fuel characteristic and        the amount of fuel having the second fuel characteristic        available for a proposed flight. This may for example, be        provided by an estimate or measurement of the amount of fuel in        the fuel tanks 53, 55 making up first and second fuel sources;    -   b) the amount of total fuel requirement for the fuel injectors        313 during pilot-only operation for the entire flight in a range        of operation in which fuel is provided from the first fuel        source 302; and/or    -   c) a fuel composition limit parameter (e.g. a certification        limit, fuel composition available for fuelling, or        aircraft/engine limits).

FIG. 26 illustrates a method 4014 of operating a gas turbine engineaccording to the example above in which fuel of a differentcharacteristic is delivered to the pilot fuel injectors during the firstengine cruise operation range. The method 4014 comprises regulating 4016fuel delivery to the pilot and/or main fuel injectors 313, 314 from thefirst fuel source 302 containing a first fuel having a first fuelcharacteristic and the second fuel source 304 containing a second fuelhaving a second fuel characteristic, as described above using the fueldelivery regulator 306 of any example herein (i.e. fuel from only thefirst fuel source 302, fuel from only the second fuel source 304, or ablend thereof). The second fuel characteristic is different from thefirst as described above.

The method further comprises switching 4018 between the pilot-only rangeof operation and the pilot-and-main range of operation at a stagingpoint during a steady state cruise mode of operation of the engine todefine a first engine cruise operation range 320 a and a second enginecruise operation range 320 b. The method further comprises delivering4020 fuel to the pilot fuel injectors 313 during at least part of thefirst engine cruise operation range 320 a having a different fuelcharacteristic from fuel delivered to one or both of the pilot and mainfuel injectors 313, 314 during the second engine cruise operation range320 b. Any of the features described above in connection with theexamples in which different fuel is supplied in the first and secondengine cruise operation ranges may be incorporated into the method 4014,even though they are not repeated here.

Varying Staging Ratio in Steady State Cruise Close to Staging Point

In another example of the present application, the staged combustionsystem 64 illustrated in FIG. 5 is additionally or alternativelyarranged to operate in a transition range of operation between thepilot-only range of operation and the pilot-and-main range of operation.In the present examples the transition range of operation is providedduring steady state cruise operation of the engine. Within thetransition range of operation, fuel is delivered to both the pilot fuelinjectors and main fuel injectors 313, 314 at a transition stagingratio. As defined elsewhere herein, the staging ratio defines therelative fuel flow mass rates attributable to the pilot fuel injectors313 compared to the main fuel injectors 314. In the pilot-only range ofoperation, the stating ratio is, by definition, 100:0. In thepilot-and-main range of operation, the combustor is configured tooperate with a pilot-and-main staging ratio which may be 20:80 or 30:40etc. Within the transition range of operation the combustor isconfigured to operate using a transition staging ratio which isdifferent from the pilot-and-main stating ratio (and different from thepilot-only staging ratio, as fuel is delivered to both the main andpilot fuel injectors during the transition range of operation). Morespecifically, in the transition range of operation, the proportion offuel flow rate attributable to the pilot fuel injectors compared to themain fuel injectors is greater than for the pilot-and-main stagingratio. In other words, a smooth or intermediate transition is providedin which a greater proportion of fuel is provided to the pilot fuelinjectors compared to switching directly between all of the fuel beingsupplied to the pilot fuel injectors in pilot-only mode, and thepilot-and-main staging ratio in which a smaller proportion of fuel issupplied to the pilot fuel injectors. In other words, rather than movingthe sharp staging point to a higher power setting (as describedelsewhere herein), the inventors have determined that a transitionregion can be included that is characterised by a slower transition(with respect to engine power setting) from the pilot-only staging ratio(100:0) to the chosen pilot-and-main staging ratio (e.g. 20:80).

The inventors have determined that by configuring the staged combustionsystem 64 to operate in a transition range (i.e. a transition range ofoperation) between the pilot-only and pilot-and-main operation theamount of CO and HC emission operating within that range of engine powersettings can be reduced. This is illustrated in FIG. 27 , which showsthe dependence of CO and HC emission index (EI) for a staged combustor(such as those described herein) on engine power setting. The transitionrange is labelled 322, and lies at an engine power range betweenpilot-only operation and pilot-and-main operation. At engine powerswithin the transition range a high level of HC and CO would be emittedif the combustor 16 were to operate at the pilot-and-main staging ratio,as shown by the dotted line in the HC and CO curve of FIG. 27 . Byoperating at the transition staging ratio (which in this case graduallyreduces as engine power increases), the CO and HC emission index isreduced, as shown by the thick solid line in the HC and CO curve withinthe transition range.

In this example, the fuel delivery regulator 306 is arranged to deliverfuel to one or both of the pilot 313 and main 314 fuel injectors duringthe transition range of operation having a different fuel characteristicfrom fuel delivered to one or both of the pilot and main fuel injectors313, 314 during at least part of the pilot-and-main range of operationThe inventors have further determined that, disadvantageously, defaultEI(nvPM) within the transition range 322 is increased for fuel of thesame characteristics. This can be seen in FIG. 28 , which illustratesthe dependence of the nvPM emission index on engine power for a stagedcombustor. The thin dotted line in FIG. 28 represents the nvPM emissionsshould the transition range 322 not be provided. The thick solid lineshows the effect on nvPM emissions of introducing the transition range322. In the present example, the inventors have determined that EI(nvPM) in the transition range 322 can be mitigated by the use of fuelof a different characteristic (e.g. SAF-rich fuel) as shown by the thickdotted line in the nvPM curve within the transition range. Thus, thenvPM penalty (required to achieve lower CO/HC emissions in thetransition range 322) is reduced by using SAF-rich fuel compositionwithin the transition range 322 rather than using a default fuelcomposition within the transition range 322.

In some examples therefore the fuel delivered to at least the pilot fuelinjectors 313 by the fuel delivery regulator 306 during the transitionrange of operation 322 may be selected to be fuel associated with alevel of nvPM production which is less than that of the fuel deliveredto one or both of the pilot and main fuel injectors 313, 314 during atleast part of the pilot-and-main range of operation. This can beachieved by the fuel contained within the first fuel source 302 beingassociated with low nvPM production compared to the fuel containedwithin the second fuel source (at corresponding combustion conditionconditions). In some examples, fuel delivered to both the pilot and mainfuel injectors 313, 314 by the fuel delivery regulator 306 during thetransition range of operation 322 is low nvPM producing fuel. In someother examples, only the fuel delivered to the pilot fuel injectors 313during the transition range of operation 322 is selected to beassociated with a low level of nvPM (i.e. with the fuel delivered to themain fuel injectors remaining the same). This may make better use of theamount of low nvPM fuel available, as supplying fuel to the main fuelinjectors (i.e. the lean part of the fuel system) may provide less of aneffect on the reduction in nvPM.

In some examples, the fuel characteristic by which fuel from the firstfuel source 302 differs from fuel from the second fuel source 304 may bethe percentage of sustainable aviation fuel (SAF) present in therespective fuel as discussed above. The fuel delivered to the pilot fuelinjectors 313 during the transition range 322 of operation wouldsimilarly have a different percentage of SAF compared to the fueldelivered to one or both of the pilot fuel injectors 313 and main fuelinjectors 314 during at least part of the pilot-and-main range ofoperation. As discussed above, compared to fossil kerosene, SAF offerssubstantially lower nvPM, and can therefore be used to mitigate theincrease in nvPM that would otherwise result from the additional cruiseoperation. For example, the fuel provided to the pilot fuel injectorsduring the transition operation range may have a greater proportion ofSAF compared to that provided to the fuel injectors during thepilot-and-main operation range. This allows a limited available amountof SAF (or other low nvPM fuel) to be used selectively to reduce overallnvPM, CO and HC emission.

This may however not always be the case. In some examples, fossilkerosene may be treated to remove aromatic components, particularlynaphthalenes, in order to produce a largely paraffinic fuel of fossilorigin which would be a low nvPM producing fuel. Other fuelcharacteristics may therefore be associated with low nvPM, such as thepercentage of aromatic content or naphthalene content. The first andsecond fuels may therefore differ by characteristics other than the SAFcontent while still achieving the desired effect on nvPM emissions.

During the transition range of operation 322 the transition stagingratio may vary with changing engine power setting. In one example, thetransition staging ratio has a continuous variation with changing enginepower within the transition range of operation 322. This may provide asmooth transition between the staging ratio in the pilot-only andpilot-and-main range of operation. The continuous variation may be suchthat the proportion of the total fuel flow to the fuel injectors (i.e.total being delivered to the pilot and main fuel injectors) that isattributable to fuel flow to the pilot fuel injectors 313 decreases withincreasing engine power during the transition range of operation 322.The proportion of the total fuel flow to the fuel injectors that isattributable to fuel flow to the main fuel injectors 314 on the contraryincreases with increasing engine power within the transition range ofoperation 322.

In other examples, the transition staging ratio may have a constantintermediate value that is different from the pilot-and-main stagingratio. The transition staging ratio may lie between the pilot-only ratioand the pilot-and-main ratio. This therefore provides a more gradualtransition from the pilot-only to pilot-and-main staging ratios. Forexample, the transition staging ratio may be 70:30, which is between100:0 in the pilot-only range, and a pilot-and-main staging ratio whichis for example 20:80 or 30:70.

In yet other examples, the transition staging ratio varies between aseries of constant intermediate values, each being different from thepilot-and-main staging ratio. Moreover, each of the intermediate ratiosmay lie between that of the pilot-only range and that of thepilot-and-main range. For example, the transition staging ratio may be aseries of values of 80:20, 60:40 and 40:60. The pilot-and-main ratio mayin this example be 20:80. The intermediate staging ratios may thusdecrease towards the pilot-and-main ratio (i.e. a progressively smallerproportion of the total fuel is provided to the pilot fuel injectors,and a progressively greater proportion of the total fuel is provided tothe main fuel injectors). This is however only one example, and anyother number and values of intermediate transition staging ratios may beused.

In some examples, the transition staging ratio may have a continuousvariation with engine power in part of the transition range of operationand may have one or more constant values in another part of thetransition range of operation. The examples above may therefore becombined. In other examples, the staging ratio may have a continuousvariation over all of the transition range of operation, or one or moreconstant values across all of the transition range of operation.

In order to supply fuel to the pilot fuel injectors 313 during thetransition operation range 322 that is different from that supplied tothe combustor 16 during the pilot-and-main range of operation the fueldelivery regulator 306 is arranged to supply fuel selectively from thefirst and second fuel sources 302, 304 as described above (e.g. fromonly the first fuel source 302, only the second fuel source 304, or amixture thereof). In one example, the fuel delivery regulator 306 maycomprise a fuel blender 318 as shown in FIG. 17 . Similarly to asdescribed above, the fuel blender is arranged to receive a supply offuel from both the first and second fuel sources 302, 304 and outputfuel from the first fuel source, fuel from the second fuel source, or ablend thereof. This blended fuel is supplied to the pilot fuel injectors313, with fuel from a single fuel source (the second fuel source 304 inthe example above) being supplied to the main fuel injectors 314. Inother examples, the blender may be arranged to supply both the main andpilot fuel injectors with an appropriate blend of fuel. In yet otherexamples, the fuel delivery regulator 306 of any of the examplesdescribed herein may be used to supply fuel in the pilot-only,transition and pilot-and-main ranges (for example, those using separateregulators for the first and second fuel sources or a pilot regulatorarranged to switch between fuel sources).

As discussed above in connection with the example shown in FIG. 17 , thedependence of the proportion of fuel from the first fuel source 302compared to that from the second fuel source 304 on fuel flow rate maybe determined according to a desired resultant level of nvPM at aparticular fuel flow rate. For example, the proportion of fuel deliveredfrom the first fuel source 302 compared to that from the second fuelsource 304 may be determined according to a desired resultant level ofnvPM at a particular fuel flow rate within the transition range 322. Theamount of fuel supplied from each source may be determined such that thenvPM does not exceed a predetermined threshold or such that the nvPMproduction is minimised over a period of operation of the gas turbineengine as described above. In any of the examples described herein thismay be achieved by suitable control of the blender 318, or any of theother fuel delivery regulators 306 described herein.

In some examples, the proportion of fuel delivered from the first fuelsource 302 compared to that delivered from the second fuel source 304during the transition operation range 322 may be determined at leastpartly according to any one or more of:

-   -   a) the amount of fuel having the first fuel characteristic and        the amount of fuel having the second fuel characteristic        available for a proposed flight. This may for example, be        provided by an estimate or measurement of the amount of fuel in        the fuel tanks 53, 55 making up first and second fuel sources;    -   b) the amount of total fuel requirement for the fuel injectors        313 during pilot-only operation for the entire flight in a range        of operation in which fuel is provided from the first fuel        source 302; and/or    -   c) a fuel composition limit parameter (e.g. a certification        limit, fuel composition available for fuelling, or        aircraft/engine limits).

FIG. 29 illustrates a method 4022 of operating a gas turbine engineaccording to the example above in which fuel of a differentcharacteristic is delivered to the pilot fuel injectors during thetransition range 322 compared to the pilot-and-main range of operation.The method 4022 comprises regulating 4024 fuel delivery to the pilotand/or main fuel injectors 313, 314 from the first fuel source 302containing a first fuel having a first fuel characteristic and thesecond fuel source 304 containing a second fuel having a second fuelcharacteristic as described above using the fuel delivery regulator 306of any example herein (e.g. such that the fuel delivered is fuel fromonly the first fuel source, fuel from only the second fuel source, or ablend thereof). The second fuel characteristic is different from thefirst as described above.

The method further comprises operating 4026 the staged combustion systemin a transition range of operation 322 between the pilot-only and thepilot-and-main ranges of operation in which fuel is delivered to boththe pilot and main fuel injectors at a transition staging ratio which isdifferent from the pilot-and-main staging ratio. The regulating 4024fuel delivery to the pilot and/or main fuel injectors comprisesdelivering 4028 fuel to one or both of the pilot and main fuel injectorsduring the transition range of operation 322 having a different fuelcharacteristic from fuel delivered to one or both of the pilot and mainfuel injectors during at least part of the pilot-and-main range ofoperation Any of the features described above in connection with theexamples in which different fuel is supplied in the transition range 322may be incorporated into the method 4022, even though they are notrepeated here.

Staged Combustor Control During Acceleration

An engine acceleration is caused by deliberately increasing fuel flowrate to a value above that necessary to maintain steady-state operation.The inventors have observed that this “over-fuelling” causes in initialincrease in fuel-air-ratio in the combustor and may give rise toincreased production of nvPM (soot or smoke), particularly at enginepower settings corresponding to pilot-only operation.

To reduce the production of excessive amounts of nvPM duringacceleration, it is known to switch to an “acceleration” mode ofoperation of a gas turbine engine in which the staging point occurs at alower engine power setting. Thus, the transition to pilot-and-mainoperation takes place at a lower engine power setting than would be thecase during steady state operation. This helps to reduce the overallamount of nvPM produced during the acceleration.

The inventors have observed that switching to such a known accelerationmode however may have a number of drawbacks. For example, an increase inHC and CO emissions may be caused. In some known solutions, maintainingacceptable CO and HC results in higher NO_(x) emissions as the leanestflames that produce the lowest NO_(x) in this region produce too much COand HC, and conversely design changes aimed at reducing CO and HC canresult in higher NO_(x) emissions over the whole power curve. Forexample one known solution could be to physically change and richen thewhole main fuel injector to reduce the CO and HC emissions, which wouldthen lead to higher NO_(x) at all powers.

In another example of the present application, the staged combustionsystem 64 illustrated in FIG. 5 is additionally or alternativelyarranged to operate in an acceleration mode of operation. In theacceleration mode the fuel delivery regulator 306 is arranged to deliverfuel to the fuel injectors (i.e. the pilot and/or main fuel injectors313, 314) having a different fuel characteristic from fuel delivered tothe fuel injectors (i.e. the pilot and/or main fuel injectors 313, 314)during at least a part of the steady state mode of operation.

The acceleration mode of the engine is one in which the fuel deliveryregulator 306 is arranged to deliver fuel to the fuel injectors 313, 314at a rate greater than that sufficient to maintain steady-stateoperation of the engine. This causes acceleration of the engine, bywhich we mean increasing the angular velocity of one or more spools ofthe engine. In the steady state mode of operation the fuel injectors arenot over-fuelled such that no engine acceleration occurs.

The inventors have determined that increased nvPM emissions when theengine operates in the acceleration mode can be avoided or reduced byusing a fuel with different characteristics from that which is usedduring steady state operation. This allows the staging point during theacceleration mode to remain the same or similar to that of the steadystate mode of operation, thus avoiding or reducing/limiting anydisadvantageous increase in HC or CO emissions.

In some examples therefore the fuel delivered to the fuel injectors(e.g. one or both of the pilot and main fuel injectors 313, 314) duringthe acceleration mode of operation may be selected to be fuel associatedwith a level of nvPM production which is less than that of the fueldelivered to the fuel injectors (e.g. one or both of the pilot and mainfuel injectors 313, 314) during at least some of the steady state modeof operation. This can be achieved by the fuel contained within thefirst fuel source 302 being associated with low nvPM production comparedto the fuel contained within the second fuel source 304 (atcorresponding combustion conditions). Fuel associated with a lower nvPMproduction may be provided to at least the pilot fuel injectors 313during operation in the acceleration mode as that will have the mostsignificant effect on nvPM production. Preferably, fuel associated witha lower nvPM production may be provided to only the pilot fuel injectorsduring the acceleration mode to make better use of a limited supply ofthat fuel.

In some examples, the fuel characteristic by which fuel from the firstfuel source 302 differs from fuel from the second fuel source 304 may bethe percentage of sustainable aviation fuel (SAF) present in therespective fuel as discussed above. The fuel delivered to the fuelinjectors during the acceleration mode of operation would similarly havea different percentage of SAF compared to the fuel delivered to one orboth of the pilot fuel injectors 313 and main 314 fuel injectors duringat least part of the steady state mode of operation. As discussed above,compared to fossil kerosene, SAF offers substantially lower nvPM, andcan therefore be used to mitigate the increase in nvPM that wouldotherwise result from over-fuelling the combustor during theacceleration mode. For example, the fuel provided to the fuel injectors313, 314 during the acceleration mode may have a greater proportion ofSAF compared to that provided to the fuel injectors during the steadystate mode. This allows a limited available amount of SAF (or other lownvPM fuel) to be used selectively to reduce overall nvPM, CO and HCemission.

As discussed above in connection with other examples, this may howevernot always be the case. In some examples, fossil kerosene may be treatedto remove aromatic components, particularly naphthalenes, in order toproduce a largely paraffinic fuel of fossil origin which would be a lownvPM producing fuel. Other fuel characteristics may therefore beassociated with low nvPM, such as the percentage of aromatic content ornaphthalene content. The first and second fuels may therefore differ bycharacteristics other than the SAF content while still achieving thedesired effect on nvPM emissions.

In some examples, the fuel delivery regulator 306 is arranged to deliverfuel having a different fuel characteristic during an acceleration modeof operation which follows a steady state mode of operation in which thecombustor system 64 is operating in pilot-only mode. In some examplestherefore, fuel characteristics are only changed for engineaccelerations that start from an engine power setting which is below thesteady-state staging point. As the power setting is below thesteady-state staging point the combustor will be operating in pilot-onlymode, and acceleration may lead to nvPM emissions increase without thechange of fuel characteristic provided by the methods of the presentapplication. In other examples however, the acceleration may be from anysteady state operation mode, whether operation in pilot-only orpilot-and-main ranges of operation.

The staged combustion system 64 may be arranged to switch between thepilot-only range of operation and the pilot-and-main ranges of operationat the same or higher engine power in the acceleration mode compared tothe steady state mode. In other words, the staged combustion system 64is controlled (e.g. by the EEC) using a staging point that is at thesame or higher engine power setting in the acceleration mode compared tothe steady state mode. This means that the staging point is not reducedto a lower engine power setting during the acceleration mode, thusavoiding or reducing any increase in HC or CO emissions, or needing torichen all of the main fuel injectors for all engine powers. Morespecifically, in some examples, the staging point may be the same inboth acceleration and steady state operation modes. The staging pointused in the acceleration mode may be referred to as an “accelerationstaging point”, whereas the staging point used during steady stateoperation may be referred to as a “cruise staging point”. In thisexample therefore, the acceleration staging point is at a power settinggreater than or equal to the cruise staging point.

In other examples, the staged combustion system may be arranged toswitch between the pilot-only range of operation and the pilot-and-mainrange of operation at a lower engine power in the acceleration modecompared to the steady state mode. The staged combustion system 64 iscontrolled (e.g. by the EEC) in this example using a staging point thatis at a lower engine power setting in the acceleration mode compared tothe steady state mode. In this example, the acceleration staging pointis greater than a default acceleration staging point according to whichthe combustion system would be controlled if fuel of a differentcharacteristic is not able to be provided to the combustor. Theacceleration staging point is therefore reduced (relative to the cruisestaging point) by an amount less than the reduction to the defaultacceleration staging point that would be found in known systems.

The fuel delivery regulator 306 may in some examples be arranged todeliver fuel to the pilot fuel injectors 313 during pilot-only operationin the acceleration mode that has a different fuel characteristic fromfuel delivered to the main fuel injectors 314 during pilot-and-mainoperation in the steady state mode of operation of the engine. In thisexample, fuel of a different characteristic (e.g. SAF rich fuel) isdelivered to the pilot fuel injectors 313 when they are operating inpilot-only mode during acceleration of the engine. Fuel that isassociated with a high nvPM (e.g. SAF poor fuel) is then provided to themain fuel injectors 314 during pilot-and-main steady state operationwhere nvPM emission is inherently lower. This allows a limited amount offuel within the first fuel source (e.g. SAF rich fuel) to be used moreeffectively by selectively using it during pilot-only engineacceleration. In this example therefore, the fuel of a different fuelcharacteristic is delivered to the pilot fuel injectors 313 in only partof the acceleration operation mode. Once the staging point is reachedwithin the acceleration mode, fuel delivery may return to the same asthat used during steady state operation (e.g. may return to using a SAFpoor fuel). In other examples, the SAF rich fuel (or fuel of anotherdifferent characteristic) may be used at all engine powers within theacceleration mode (e.g. in pilot-only and pilot-and-main operation).

In one example, the fuel delivery regulator 306 is arranged to deliverfuel during the pilot-only range of operation in the acceleration modehaving a fuel characteristic determined based on a fuel-to-air ratio inthe combustor 16. In this example, as the fuel-to-air ratio decreases,the proportion of fuel associated with low nvPM production delivered tothe pilot fuel injectors may also be decreased. As the fuel-to-air ratioreduces (e.g. because as the engine spool speed increases so the airmass flow within the combustor also increases) the amount of a low nvPMproducing fuel (e.g. a SAF rich fuel) may also be reduced. This allowsthe low nvPM producing fuel to be used during a part of the accelerationmode of operation in which its benefits are greater, thus making betteruse of a limited availability of that fuel. In other examples, the fuelcharacteristics may be determined based on another suitable controlparameter that is linked to combustor nvPM formation other than thefuel-to-air ratio.

In other examples, the fuel delivery regulator 306 is arranged to switchdelivery of fuel to the fuel injectors (one or both of the main andpilot fuel injectors 313, 314) to that having a different fuelcharacteristic at a start point of a period of operation in theacceleration mode. In this example therefore, the fuel composition maybe switched to an alternative composition (e.g. switched to a SAF richfuel) at the start of a period of acceleration. In some examples, thefuel regulator 306 may continue to deliver the same fuel throughout theacceleration mode of operation. In other examples, the fuel deliveryregulator 306 is arranged to return to delivery of fuel having the samefuel characteristic as that delivered in the steady state mode followinga transition to pilot-and-main operation. The fuel composition istherefore switched back to the default steady state fuel characteristicsonce the staging point has been passed. This may also help to restrictthe use of fuel associated with low nvPM emissions to only part of theacceleration mode, thus making more effective use of a limited supply ofthat fuel.

In order to supply fuel to the pilot and/or main fuel injectors 313, 314during the acceleration mode of the combustion system that is differentfrom that supplied to the combustor during the steady state mode ofoperation the fuel delivery regulator 306 is arranged to supply fuelselectively from the first and second fuel sources 302, 304 as describedabove. In one example, the fuel delivery regulator 306 may comprise afuel blender 318 as shown in FIG. 17 . Similarly to as described above,the fuel blender is arranged to receive a supply of fuel from both thefirst and second fuel sources 302, 304 and output fuel from the firstfuel source 302, fuel from the second fuel source 304, or a blendthereof. This blended fuel is supplied to the pilot fuel injectors 313,with fuel from a single fuel source (the second fuel source 304 in theexample above) being supplied to the main fuel injectors 314. In otherexamples, the blender may be arranged to supply both the main and pilotfuel injectors 313, 314 with an appropriate blend of fuel. In yet otherexamples, the fuel delivery regulator 306 of any of the examplesdescribed herein may be used to supply fuel during at least part of theacceleration mode (to either or both of the pilot and main fuelinjectors) that is different from that delivered during at least part ofthe steady state mode of operation (for example, those using separateregulators for the first and second fuel sources or a pilot regulatorarranged to switch between fuel sources).

As discussed above in connection with the example shown in FIG. 17 , thedependence of the proportion of fuel from the first fuel source 302compared to that from the second fuel source 304 on fuel flow rate maybe determined according to a desired resultant level of nvPM at aparticular fuel flow rate. For example, the proportion of fuel deliveredfrom the first fuel source 302 compared to that from the second fuelsource 304 may be determined according to a desired resultant level ofnvPM at a particular fuel flow rate during the acceleration mode. Theamount of fuel supplied from each source may be determined such that thenvPM does not exceed a predetermined threshold or such that the nvPMproduction is minimised over a period of operation of the gas turbineengine as described above. In any of the examples described herein thismay be achieved by suitable control of the blender 318, or any of theother fuel delivery regulators 306 described herein.

In some examples, the proportion of fuel delivered from the first fuelsource 302 compared to that delivered from the second fuel source 304during the acceleration mode of the combustion system may be determinedat least partly according to any one or more of:

-   -   a) the amount of fuel having the first fuel characteristic and        the amount of fuel having the second fuel characteristic        available for a proposed flight. This may for example, be        provided by an estimate or measurement of the amount of fuel in        the fuel tanks 53, 55 making up the first and second fuel        sources 302, 304;    -   b) the amount of total fuel requirement for the pilot fuel        injectors 313 during pilot-only operation for the entire flight        in a range of operation in which fuel is provided from the first        fuel source 302; and/or    -   c) a fuel composition limit parameter (e.g. a certification        limit, fuel composition available for fuelling, or        aircraft/engine limits).

FIG. 30 illustrates a method 4030 of operating a gas turbine engineaccording to the example above in which fuel of a differentcharacteristic is delivered to the fuel injectors (e.g. one or both ofthe pilot and main fuel injectors 313, 314) during at least part of theacceleration mode compared to during at least part of the steady statemode. The method 4030 comprises regulating 4032 fuel delivery to thepilot and/or main fuel injectors 313, 314 from the first fuel source 302containing a first fuel having a first fuel characteristic and from thesecond fuel source 304 containing a second fuel having a second fuelcharacteristic, as described above using the fuel delivery regulator 306of any example herein. The second fuel characteristic is different fromthe first as described above.

The method further comprises operating 4034 the staged combustion system64 in an acceleration mode in which acceleration of the engine from asteady state mode of operation is caused. The method 4030 furthercomprises delivering 4036 fuel to the fuel injectors (e.g. one or bothof the pilot and main fuel injectors 313, 314), during at least part ofoperation in the acceleration mode, having a different fuelcharacteristic from fuel delivered to the fuel injectors (e.g. one orboth of the pilot and main fuel injectors 313, 314) during at least apart of the steady state mode of operation of the engine.

Any of the features described above in connection with the examples inwhich different fuel is supplied in the acceleration mode compared toduring steady state cruise mode of operation may be incorporated intothe method 4030 of FIG. 30 , even if they are not repeated here.

Staging Point Determination Based on Fuel Characteristics

In other examples of the present application, the staging pointaccording to which the staged combustion system 64 is operated may bedetermined based on a determination of the characteristics of the fuelwith which it is provided. The inventors have determined that thestaging point can be based on the characteristics of the fuel to makeadvantageous use of the particular fuel being provided to the engine 10.

In some examples, the aircraft illustrated in FIG. 4 may be arranged tohave only a single fuel source. In such an example, the fuel tanks 53,55 may be fluidly coupled to form a single fuel source on board theaircraft. The aircraft may therefore carry fuel having the same fuelcharacteristics, rather than fuels having different characteristics.

FIG. 31 illustrates an example of a staged combustion system 64 havingfeatures corresponding to that of FIG. 5 , but which is provided withfuel from a single source aboard the aircraft 1. In this example, fuelis received by the fuel delivery regulator 306 via a fuel pump 308, froma single fuel source, e.g. fuel tanks 53, 55 shown in FIG. 4 . Featurescommon with the example of FIG. 5 are labelled accordingly, and so willnot be described again.

In the example illustrated in FIG. 31 , the gas turbine engine 10further comprises a fuel characteristic determination module 330. Thefuel characteristic determination module 330 is configured to determineone or more characteristics of the fuel being supplied to the fueldelivery regulator 306. In the described example, the fuelcharacteristic determination module 330 is in communication with asensor device 332, which is configured to perform a measurement on fuelflowing to the fuel delivery regulator 306 in order to determine fuelcharacteristics. The sensor device 332 may take a number of differentforms, and may operate according to any of the examples of thedetermination of fuel characteristics disclosed herein. In otherexamples, the fuel characteristic determination module 330 may receivesignals from a sensor device located elsewhere on board the aircraftwhich is configured to perform a measurement of fuel characteristics. Inyet other examples, the fuel characteristic determination module 330 mayobtain fuel characteristics from sources other than a sensor device asdescribed elsewhere herein, for example the fuel characteristics may bereceived via a data communication channel, or from a user input.

The EEC 42 (which may be referred to more generally as “a controller”)is in communication with the fuel characteristic determination module330 such that it may receive the fuel characteristics of the fuel beingsupplied to the fuel delivery regulator 306. In the present example, thefuel characteristic determination module 330 is shown separately fromthe EEC, but in other examples they may be combined. The controller 42is configured to determine the staging point at which the stagedcombustion system is switched between its pilot-only operation andpilot-and-main operation. The staging point is determined based on theone or more fuel characteristics. Once the staging point is determinedin this way it is used by the controller 42 to control operation of thestaged combustion system 64, e.g. it is used to control operation of thefuel delivery regulator so that appropriate flow of fuel is provided tothe pilot manifold 309, or the pilot 309 and the main manifold 310 foroperation in the pilot-only and pilot-and-main modes respectively.

The inventors have determined that the staged combustion system 64 canbe advantageously controlled based on the characteristics of the fuelwhich it is being supplied. In particular, the staging point can bechosen to make advantageous use of the characteristics of the fuel beingsupplied to the engine.

In one example, the staging point is determined based on the one or morefuel characteristics indicating that the fuel is associated with a lownvPM production level (e.g. low compared to fossil kerosene, atcorresponding combustion conditions). This may allow the staging pointto be adjusted so that it corresponds to an engine operating conditionthat would otherwise lead to high levels of nvPM production. Forexample, as illustrated in FIGS. 23 and 25 , if a low nvPM producingfuel is determined to be being supplied to the combustor the stagingpoint may be adjusted to reduce CO and HC production, without causing adisadvantageous increase in nvPM production which would otherwise occurif a relatively high nvPM producing fuel was being used.

In some examples, the fuel characteristic on which the staging pointdetermination is based may be the percentage of sustainable aviationfuel (SAF) present in the respective fuel. As discussed above, comparedto fossil kerosene, SAF offers substantially lower nvPM, and cantherefore be used to mitigate changes in the staging point that wouldotherwise increase nvPM production.

In some examples, fossil kerosene may be treated to remove aromaticcomponents, particularly naphthalenes, in order to produce a largelyparaffinic fuel of fossil origin which would be a low nvPM producingfuel. Other fuel characteristics may therefore be associated with lownvPM, such as the percentage of aromatic content or naphthalene content.In other examples, the one or more fuel characteristics on which thestaging point is determined may include an aromatic hydrocarbon contentof the fuel, and/or a naphthalene content of the fuel. Thesecharacteristics may also indicate the level of nvPM that will beproduced by the fuel, and allow the staging point to be determinedaccordingly.

The controller 42 may be configured to determine the staging point suchthat the staging point associated with fuel that is a low nvPM producingfuel corresponds to a higher engine power setting compared to thestaging point associated with one or more fuel characteristics thatindicate that the fuel is associated with a relatively higher nvPMproduction. In other words, the staging point may be increased to ahigher engine power for a fuel which is associated with lower nvPMproduction compared to a fuel associated with a higher nvPM production.The engine power at which the staging point occurs may therefore beincreased with decreasing nvPM production of the fuel. As discussedabove in connection with FIGS. 23 and 25 this increase in the stagingpoint may help to reduce CO and HC production, without a significantincrease in nvPM. The low nvPM producing fuel may be one that produces alower nvPM emission compared to fossil kerosene at correspondingcombustion conditions. The low nvPM fuel may be a SAF rich fuel, whichhas at least some SAF content, and preferably a SAF content of greaterthan 10%, or more preferably equal to or greater than 50%.

In some examples, the staging point determined based on the fuelcharacteristics may be a cruise staging point with which the combustionsystem 64 is controlled during steady state cruise operation of theengine. The controller 42 may be configured to determine the stagingpoint so that it corresponds to an engine power setting that results ina switch between pilot-only and pilot-and-main operation during steadystate cruise. The staging point determined by the controller 42 based onthe fuel characteristics may therefore create a boundary between a firstengine cruise operation range and a second engine cruise operationrange. The staging point may be selected to define these two cruiseoperation points if it is determined that a relatively low nvPM fuel isbeing used (e.g. a high SAF, and/or low aromatic and/or low naphthalenefuel). This may allow pilot-only operation during cruise to makeadvantageous use of its low HO and CO production, while avoiding theincrease in nvPM that would otherwise occur if a high nvPM fuel werebeing supplied by the engine (see FIGS. 27 and 28 , and associateddescription above).

The first cruise operation range may correspond to operation of theaircraft in a later part of a cruise segment of a flight, and the secondoperation range may correspond to operation of the aircraft in arelatively earlier part of the cruise segment. As discussed above, thismay allow the staged combustion system 64 to switch to pilot-onlyoperation during a later part of a cruise segment (e.g. a segment ofcruise at a constant altitude). In another example, the first cruiseoperation range may correspond to steady state subsonic cruise operationof the engine and the second cruise operation range may correspond tosteady state supersonic cruise operation of the engine. In both of theseexamples, the staging point is selected so that pilot-only operationoccurs at low engine power cruise operation, for example either in alater part of a cruise segment or during subsonic cruise. The steadystate supersonic cruise operation may occur before the steady statesubsonic cruise operation, or vice versa. In some examples, the steadystate subsonic cruise operation may be subsonic cruise operation overland, while the steady state supersonic cruise operation may besupersonic cruise operation over water.

In another example, the staging point determined according to the fuelcharacteristics may be an engine acceleration staging point according towhich the staged combustion system is controlled during an accelerationcondition of the engine. In such an example, the controller may beconfigured to select the engine acceleration staging point so that it isthe same as that used during cruise. Specifically, this may be done ifit is determined that a low nvPM fuel is being used (e.g. a high SAF,and/or low aromatic and/or low naphthalene fuel). As discussed above,this may allow acceleration to occur (at least partly) while remainingin pilot-only mode despite the increase in fuel flow rate to provideacceleration. This may help to avoid increase in HC and CO productionthat would otherwise occur by increasing the staging point during engineacceleration.

In some examples, the controller 42 may be arranged to determine one orboth of the cruise and acceleration staging point, and may be arrangedto determine staging points for both supersonic/subsonic operation andfor earlier/later parts of a cruise segment.

FIG. 32 illustrates a method 4038 of operating a gas turbine engine foran aircraft. The method may be performed by the apparatus of FIG. 31 .The method 4038 comprises determining 4040 one or more fuelcharacteristics of a fuel being supplied to the combustion system;determining 4042 a staging point defining the point at which thecombustion system is switched between pilot-only operation andpilot-and-main operation based on the determined one or more fuelcharacteristics; and controlling 4044 the staged combustion systemaccording to the determined staging point.

The one or more fuel characteristics may indicate that the fuel isassociated with low nvPM production level compared to fossil kerosene asdiscussed above. The one or more fuel characteristics include any one ormore of: (i) a percentage of sustainable aviation fuel in the fuel; (ii)an aromatic hydrocarbon content of the fuel; and/or (iii) a naphthalenecontent of the fuel. Other fuel characteristics may be used.

Determining 4042 the staging point may comprise determining 4046 thestaging point such that the staging point associated with one or morefuel characteristics that indicate that the fuel is associated with alow nvPM production corresponds to a higher engine power settingcompared to the staging point associated with one or more fuelcharacteristics that indicate that the fuel is associated with arelatively higher nvPM production.

The staging point determined 4042 by the method of FIG. 32 may be acruise staging point, and may be used during a later part of a cruisesegment of a light, or during subsonic cruise operation of a supersoniccapable aircraft as discussed in the various examples above.

Any of the features described above in connection with the examples inwhich the staging point is determined according to one or more fuelcharacteristics may be incorporated into the method 4038 of FIG. 32 .

In the examples above, the staging point is determined according to thenvPM production characteristics of the fuel. This is however only oneexample, and other fuel characteristics may be taken into account inorder to determine a suitable staging point in order to take advantageof the characteristics of the fuel being supplied to the combustor.

Staging Ratio Determination Based on Fuel Characteristics

In other examples, the controller 42 illustrated in FIG. 31 isadditionally, or alternatively, arranged to determine a staging ratioaccording to the one or more fuel characteristics. As discussedelsewhere herein, the controller 42 is configured to control the fueldelivery regulator 306 (and hence the staged combustion system 64)according to a staging ratio. The staging ratio defines the ratio ofpilot fuel injector 313 fuel flow to the main fuel injector 314 fuelflow. The inventors have determined that the staging ratio can be chosento make advantageous use of certain fuel characteristics of fuel that isbeing provided to the fuel delivery regulator by intelligently choosingthe ratio based on the fuel characteristics.

For example, the staging ratio may be determined based on the one ormore fuel characteristics indicating that the fuel is associated with alow nvPM production level (e.g. low compared to fossil kerosene, atcorresponding combustion conditions). This may allow the staging ratioto be adjusted to reduce CO and HC production in a way that wouldotherwise lead to high levels of nvPM production as discussed in variousexamples above.

In some examples, the fuel characteristic(s) on which the staging ratiodetermination is based may be the percentage of sustainable aviationfuel (SAF) present in the respective fuel. As discussed above, comparedto fossil kerosene, SAF offers substantially lower nvPM, and cantherefore be used to mitigate changes in the staging ratio that wouldotherwise increase nvPM production.

In some examples, fossil kerosene may be treated to remove aromaticcomponents, particularly naphthalenes, in order to produce a largelyparaffinic fuel of fossil origin which would be a low nvPM producingfuel. Other fuel characteristics may therefore be associated with lownvPM, such as the percentage of aromatic content or naphthalene content.In other examples, the one or more fuel characteristics on which thestaging ratio is determined may include an aromatic hydrocarbon contentof the fuel, and/or a naphthalene content of the fuel. Thesecharacteristics may also indicate the level of nvPM that will beproduced by the fuel, and allow the staging ratio to be determinedaccordingly.

The controller 42 may be configured to determine a transition stagingratio which allows the staged combustion system 64 to be operated in atransition range of operation between the pilot-only range of operationand pilot-and-main range of operation as discussed above in connectionwith FIGS. 27 and 28 . The transition staging ratio may be determinedaccording to the one or more fuel characteristics indicating that thefuel being supplied to the fuel delivery regulator 306 is associatedwith a low nvPM production level (e.g. according to the SAF content,aromatic content or naphthalene content; low nvPM production beingrelatively less than for fossil kerosene fuel). The transition stagingratio may be different to a pilot-and-main staging ratio according towhich the controller 42 controls the staged combustion system duringpilot-and-main operation. The pilot-and-main staging ratio may be adefault ratio, and may be determined according to known techniques.

As discussed above in connection with FIGS. 27 and 28 , by configuringthe staged combustion system 64 to operate in the transition rangebetween the pilot-only and pilot-and-main operation the amount of CO andHC emission operating within that range of engine power settings can bereduced. By detecting that the combustor is being provided with a fuelassociated with low nvPM production, the controller may determine thatthe transition staging ratio can be used to reduce CO and HC emissions,without causing an excessive increase in nvPM production.

Any of the features of the transition staging ratio discussed above maybe incorporated into the presently described examples in which thetransition staging ratio is selected according to the one or more fuelcharacteristics. For example, during the transition phase the transitionstaging ratio may vary with changing engine power setting. In oneexample, the transition staging ratio has a continuous variation withchanging engine power within the transition range of operation. This mayprovide a smooth transition between the staging ratio in the pilot-onlyand pilot-and-main range of operation. The continuous variation may besuch that the proportion of the total fuel flow to the fuel injectors(i.e. total being delivered to the pilot and main fuel injectors) thatis attributable to fuel flow to the pilot fuel injectors 313 decreaseswith increasing engine power during the transition range of operation.The proportion of the total fuel flow to the fuel injectors that isattributable to fuel flow to the main fuel injectors 314 on the contraryincreases with increasing engine power within the transition range ofoperation.

In other examples, the transition staging ratio has a constantintermediate value that is different from the pilot-and-main stagingratio. The transition staging ratio may lie between the pilot-only ratioand the pilot-and-main ratio. This therefore provides a more gradualtransition from the pilot-only to pilot-and-main staging ratios. Forexample, the transition staging ratio may be 70:30, which is between100:0 in the pilot-only range, and a pilot-and-main staging ratio whichmay be 20:80 or 30:70.

In yet other examples, the transition staging ratio varies between aseries of constant intermediate values, each being different from thepilot-and-main staging ratio. Moreover, each of the intermediate ratiosmay lie between that of the pilot-only range and that of thepilot-and-main range. For example, the transition staging ratio may varybetween a series of values of 80:20, 60:40 and 40:60. The pilot-and-mainratio may be 20:80 in this example. The intermediate staging ratios maythus decrease towards the pilot-and-main ratio (i.e. a progressivelysmaller proportion of the total fuel is provided to the pilot fuelinjectors 313, and a progressively greater proportion of the total fuelis provided to the main fuel injectors 314). This is however only oneexample, and any other number and values of intermediate transitionstaging ratios may be used.

In some examples, the transition staging ratio may have a continuousvariation with engine power in part of the transition range of operationand may have one or more constant values in another part of thetransition range of operation. The examples above may therefore becombined. In other examples, the staging ratio may have a continuousvariation with engine power over all of the transition range ofoperation, or may have one or more constant values with engine poweracross all of the transition range of operation.

FIG. 33 illustrates an example of a method 4050 of operating a gasturbine engine. The method 4050 comprises: determining 4052 one or morefuel characteristics of a fuel being supplied to the staged combustionsystem 64; determining 4054 a staging ratio defining the ratio of pilotfuel injector fuel flow to main fuel injector fuel flow; and controlling4056 the staged combustion system 64 according to the determined stagingratio. The method 4050 may be carried out by the apparatus shown in FIG.31 (additionally or alternatively to the method 4038 in which a stagingpoint is determined).

The one or more fuel characteristics may indicate that the fuel isassociated with low nvPM production level compared to fossil kerosene asdiscussed above. The one or more fuel characteristics include any one ormore of: (i) a percentage of sustainable aviation fuel in the fuel; (ii)an aromatic hydrocarbon content of the fuel; and/or (iii) a naphthalenecontent of the fuel. Other fuel characteristics may be used.

The controlling 4056 of the staged combustion system 64 may comprisecontrolling 4058 the staged combustion system 64 during thepilot-and-main range of operation according to a pilot-and-main stagingratio as discussed above. The determining 4054 of the staging ratio maycomprise determining 4060 a transition staging ratio. Controlling 4056the staged combustion system 64 may then comprise controlling 4062 it sothat it is operated in a transition range of operation between thepilot-only range of operation and the pilot-and-main range of operationas discussed above. The transition staging ratio may be determinedaccording to any of the examples given above.

Any of the features described above in connection with the examples inwhich the staging ratio is determined according to one or more fuelcharacteristics may be incorporated into the method 4050 of FIG. 33 .

In the examples above, the staging ratio is determined according to thenvPM production characteristics of the fuel. This is however only oneexample, and other fuel characteristics may be taken into account inorder to determine a suitable staging point in order to take advantageof the characteristics of the fuel being supplied to the combustor.

Abnormal Operating Conditions

The regulation of the fuel delivery according to any of the examplesdescribed herein is understood to be suitable for operating under normalrunning conditions of the gas turbine engine 10 or the aircraft 1.During abnormal operation conditions the delivery of fuel to thecombustor 16 as described herein (e.g. to control nvPM) may beoverridden. The fuel delivery regulation described herein is thereforeapplicable during at least part of the operation of the associated gasturbine engine, e.g. where fuel availability, either due to tankcapacity limits or unexpected abnormal running conditions, does notprevent it.

For example, in abnormal operation, such as following failure of anengine, a requirement to transfer fuel from one wing tank to anotherwing tank (in order to maintain an acceptable aircraft centre of masslateral position) may override fuel delivery regulation according to thepresent application, particularly in cases requiring fuel transfer fromthe first fuel source 302 to the second fuel source 304 or vice versa.However, if the two wing fuel tanks both form part of the same fuelsource (i.e. both are part of the first fuel source 302 or both are partof the second fuel source 304) and are fluidly interconnected eitherdirectly or via one or more further fuel tanks that are also part of thesame fuel source (the first or second fuel source respectively) then thefuel regulation described herein may continue to operate despite suchabnormal conditions.

Where the fuel delivery regulator 306 is arranged to switch betweensources or blend fuel from the first and second fuel sources 302, 304the switching or blending may be overridden if one of the fuel sourcesis depleted because of a leak or other fuel unexpected reasons (e.g.incorrect fuel loading). In some or all examples, in particular thatshown in FIG. 9 , a fuel bypass may be provided in which fuel can bedistributed between the first and second fuel sources in the event of anabnormal operating condition. For example, in the example of FIG. 9 afuel bypass may be provided upstream of the fuel delivery regulator 306,or between fuel tanks of the first and second fuel sources 302, 304,which provides an emergency interconnection between the first and secondfuel sources. The pilot regulator 306 a and main regulator 306 b aretherefore capable of being suppled with fuel from either of the firstand second fuel sources 302, 304 should one of the fuel sources failduring operation.

The fuel delivery regulation may further be overridden during part of amission of the aircraft if insufficient amounts of the first and secondfuel can be contained within the aircraft fuel tanks. For some missions,the total fuel loading requirement for a proposed flight may dictate theminimum extent to which each fuel tank must be filled, and this mayover-ride certain other means of fuel delivery control in the variousexamples described herein.

Calculating Fuel Allocation

The present application further provides a method of determining a fuelallocation for an aircraft. The method allows the determination of afuel allocation according to which fuel is loaded onto the aircraft 1 tocarry out a proposed flight or mission. The aircraft for which themethod is used may be that illustrated in FIG. 4 , which comprises afirst fuel source 302 adapted to contain a first fuel having a firstfuel characteristic and a second fuel source 304 adapted to contain asecond fuel having a second fuel characteristic, the second fuelcharacteristic being different from the first. As discussed above, theaircraft 1 comprises one or more gas turbine engines 10 powered by fuelfrom the first and second fuel sources 302, 304. The gas turbine engines10 each comprise a fuel delivery regulator 306 arranged to supply fuelfrom each fuel source, or a blend thereof, and a staged combustionsystem 64, as illustrated in FIG. 5 , or as described anywhere herein.

A method 4070 of determining a fuel allocation is illustrated in FIG. 34. The method 4070 comprises obtaining 4072 a proposed missiondescription comprising a list of operating points for the gas turbineengine(s) 10 of the aircraft 1 during an operating mission. The list ofoperating points includes information on the operation of the gasturbine engines 10 of the aircraft 1 that are expected for a particularplanned period of operation for which fuel is to be loaded onto theaircraft. The list of operating points may include a variety ofinformation from which the expected nvPM impact of the gas turbineengine during each part of the operating mission can be determined forfuels of different characteristics being used. The operating points ofthe mission description may include any one or more of: one or moreconditions in which the gas turbine engines 10 are to operate (e.g.location and/or ambient conditions expected for the specific mission),one or more fuel flow rate values corresponding to an operating point,and a time duration of operation at a corresponding operating point. Theoperating points may therefore indicate that the mission, for example,comprises a period of operating the engines at a cruise operatingcondition, in certain ambient conditions, and in which a specified fuelflow rate is required. Any other suitable information can be provided inthe list of operating points so that the nvPM production for the variousparts of the flight can be found for given fuel characteristics. Themission description may include details of the operation of the aircrafton the ground in order to complete the operating mission.

The method 4070 further comprises obtaining 4074 nvPM impact parametersfor the gas turbine engines 10 based on the obtained missiondescription. The impact parameters are associated with each of theoperating points of the proposed mission, and may define an amount ofnvPM produced by the gas turbine engines 10 for different respectivefuel compositions comprising the first fuel, the second fuel or a blendthereof at each operating condition of the mission description.

The amount of nvPM produced by the engine may be determining using alookup table of nvPM number as a function of WE for all operatingconditions (e.g. taking into account different positions of the stagingpoint at different operating points of the mission), for different fuelcharacteristics. The variation with fuel characteristics may, in someexamples, be parameterised by percentage SAF content. The lookup tableused to determine the nvPM production may be chosen from a set of lookuptables corresponding to different types of SAF (HEFA, ATJ etc). WherenvPM production is similar for different fuels the same lookup table maybe used for each.

The impact parameters may be the nvPM impact parameters describedelsewhere herein and may relate to the cost or harm of nvPM (e.g. soot)emission of a particular type or in a certain situation (in addition oralternatively to simply indicating an amount of nvPM produced). The nvPMimpact parameters may therefore include any one or more of:

-   -   i) height above ground level at which the nvPM production takes        place;    -   ii) position (e.g. location e.g. longitude and latitude) of the        nvPM production;    -   iii) weather/atmospheric conditions at a location of the nvPM        production;    -   iv) climate impacts associated with location of the nvPM        production;    -   v) mass/size of the individual nvPM particles produced;    -   vi) potential contrail production and/or contrail        characteristics;    -   vii) local air quality (LAQ) impact of nvPM production; and/or    -   viii) amount of nvPM produced (e.g. mass/number)

The method 4070 further comprises calculating 4076 an optimised set ofone or more fuel characteristics for each operating point of theproposed mission defined in the mission description based on the nvPMimpact parameters. In this step, the method calculates the fuelcharacteristics for each part of the proposed mission which give anoptimal set of nvPM impact parameters. Calculating the optimised set ofone or more fuel characteristics comprises minimising a cost functiondependent on the one or more nvPM impact parameters. In some examples,the cost function may take into account only the amount of nvPM producedduring each part of the mission such that it can be minimised. In otherexamples, more complex cost functions can be defined as describedelsewhere herein to take into account other factors that relate to theimpact of the nvPM production (e.g. using the other impact parametersdefined above).

Once the optimised set of fuel characteristics has been calculated, themethod 4070 comprises determining 4078 a fuel allocation based on theoptimised set of one or more fuel characteristics. The fuel allocationdefines how fuel is allocated to the mission and therefore how fuel isto be loaded onto the aircraft to meet the needs of the optimised fuelcharacteristics over the duration of the mission. The fuel allocationmay include any one or more of:

-   -   i) an amount of fuel (e.g. volume or mass) allocated to each of        the first and second fuel sources. This may allow the required        amount of fuel to be loaded during a refuelling process in which        the aircraft is connected to a fuel source such as a fuel tanker        or fuel supply line;    -   ii) the first fuel characteristic and/or the second fuel        characteristics; and/or    -   iii) a fuel blending ratio (e.g. a ratio of a default or        non-default fuel).

By specifying amount and the characteristics of the fuel, the requiredfuel can be loaded from the different types of fuel that might beavailable. In some examples, fuel may be blended from different fuelsavailable before being loaded e.g. blended from a default andnon-default fuel as described later.

The method 4070 may further comprise determining one or more fuel usageparameters corresponding to the fuel allocation, the fuel usageparameters defining how the fuel is to be used during the missiondefined by the mission description. The fuel usage parameters may definehow the optimised fuel characteristics required for each part of themission are to be provided to the combustor 16 of the respective engine.The fuel usage parameters may be combined to form a “mission fuel usage”which defines how the fuel is used over the duration of the mission. Thefuel usage parameters may be provided to the aircraft 1 so that the gasturbine engine 10 (e.g. the fuel delivery regulator 306) can becontrolled accordingly, or the fuel tanks 53, 55 configured as required.The one or more fuel usage parameters may include any one or more of:

-   -   i) a blending schedule according to which fuel from the first        fuel source 302 and the second fuel source 304 is blended by the        fuel delivery regulator 306 (e.g. using the fuel blender 318        described above);    -   ii) a switching schedule according to which the fuel delivery        regulator 306 is configured to switch between delivery of fuel        from the first fuel source 302 and the second fuel source 304;    -   iii) an onboard fuel blending ratio according to which the fuel        delivery regulator 306 is configured to blend fuel from the        sources on board the aircraft;    -   iv) an allocation of fuel tanks 53, 55 provided in the aircraft        to form the first fuel source 302 and the second fuel source        304. This may allow the allocation of fuel tanks to be        configured so that the amount of fuel of each type required for        the flight can be stored on board the aircraft; and/or    -   ii) an isolation valve setting for fuel tanks 53, 55 forming the        first fuel source 302 and the second fuel source 304. This may        allow the configuration of the fuel tanks to be configured by        determining which tanks on board the aircraft are isolated or in        fluidic communication with each other.

The inventors have determined that by calculating the fuel allocation inthis way, fuel can be loaded onto the aircraft such that there is therequired amount of fuel, having the required characteristics, on boardthe aircraft for it to carry out the proposed mission while reducing thenvPM impact. This may allow better use of the characteristics of thefuel available in reducing nvPM compared to loading a set amount of thetypes of fuel available. It may also ensure that adequate fuel isavailable in order to carry out the methods of combustor controldescribed herein in which fuel of different characteristics isintelligently provided to the staged combustion system 64 duringdifferent operating conditions.

The first fuel characteristic may be associated with a level of nvPMproduction which is less than that of the second fuel characteristic(under corresponding combustion conditions). More specifically, thefirst fuel characteristic and the second fuel characteristic may be apercentage of SAF present in the respective fuel. As discussed elsewhereherein, the percentage of SAF may affect the level of nvPM production.The first and second fuels may differ by other characteristics,including any of those defined herein. For example, they may differ byaromatic content (or naphthalene content).

In some examples, the aircraft may be loaded using a choice of differentfuels which are available at the location at which it is being fuelled.The type and quantity of fuel available may be different betweendifferent locations. The present method may therefore allow an optimisedamount of fuel of the types available to be loaded onto the aircraft bytaking into account the characteristics of the fuel available and thequantity available. In some examples, the fuel available may be a“default fuel” and a “non-default fuel”. The default fuel may be widelyavailable fuel which is primarily fossil kerosene (e.g. it may be Jet Aor JetA-1). The default fuel may include a low percentage of SAF. Thedefault fuel therefore corresponds to fuel having the second fuelcharacteristics discussed elsewhere herein. It may be considered to beavailable in volumes that are not subject to an upper limit. Thenon-default fuel may be a less widely available fuel, and includes arelatively higher SAF content compared to the default fuel. Thenon-default fuel may comprise 50% or more SAF. It may comprise otherpercentages of SAF which are significantly greater than the default fuel(the remainder being fossil kerosene), and may be 100% SAF.

The optimised set of one or more fuel characteristics for each flightcondition may be further determined based on any one or more of:

-   -   i) the achievable range of fuel compositions that can be        provided by the fuel delivery regulator 306 of the engine 10 of        the aircraft 1 for which the fuel is being loaded. For example,        the fuel delivery regulator 306 may be arranged to deliver a        blend of fuel from the first and second fuel sources 302, 304,        switch between fuel from the first and second fuel sources, or        deliver exclusively fuel from the first fuel source 302 or fuel        from the second fuel source 304 to the main 314 or pilot 313        fuel injectors throughout the mission. The type of regulator may        therefore restrict the characteristics of the fuel that can be        supplied to the combustor, and so can advantageously be taken        into account when calculating the fuel characteristics;    -   ii) a total quantity of non-default fuel allocated to the        mission. The optimisation of the required fuel characteristics        may take into account the amount of the non-default type of fuel        available at the location at which refuelling is taking place.        As the supply of the non-default fuel may be limited it can be        taken into account when optimising the fuel required of each        type available;    -   iii) a total fuel requirement for the mission. For example, the        total amount of fuel required to complete the mission (including        any contingency amount) may also be taken into account;    -   iv) the capacities of the fuel tanks of the aircraft. This may        allow the amount of each fuel that can be stored by the tanks of        the aircraft to be taken into account. For example, some        aircraft may have a fixed configuration of fuel tanks which can        hold a predefined amount of each type of fuel available; and/or    -   v) restrictions on how the aircraft fuel tanks can be allocated        to the first or the second fuel source. As discussed above, some        aircraft may have a configurable set of fuel tanks which can        provide flexibility in the amount of fuel of each type that can        be stored on board the aircraft.

The method 4070 may be part of a method 4080 of loading fuel onto anaircraft 1. Such a method is illustrated in FIG. 35 . The method 4080may comprise determining 4082 a fuel allocation using the method 4070described above. Once the fuel allocation is determined, the method 4080comprises loading 4084 fuel onto the aircraft according to the fuelallocation. This may comprise loading fuel by connecting the aircraft 1to a fuel supply using a known method (e.g. as illustrated in FIG. 4 ),and may further comprise configurating any fuel tanks as required,selecting between fuels of different characteristics, and loading therequired mass or volume of each fuel. The step of loading 4084 the fuelmay also comprise storing any required control parameters in a controlsystem of the relevant engine (e.g. the EEC 42) or the aircraft. Thecontrol parameters may, for example, include the fuel usage parametersdescribed above.

The method 4070 is a computer implemented method. In some examples, themethod 4070 may be performed by a computing device located on board theaircraft 1, such as a control system of the aircraft (e.g. the EEC 42 oranother control system provided on the engine or aircraft). The method4070 may be implemented by any suitable computing device, either onboard the aircraft 1, separate from the aircraft 1 as part of a fuelloading system, or as a dedicated system.

FIG. 36 illustrates a fuel allocation determination system 5000 fordetermining a fuel allocation. The system 5000 may carry out the method4070 described above. Any feature disclosed above in connection with themethod 4070 may also apply to the system 5000. The system 5000 comprisesa mission description obtaining module 5002 configured to obtain aproposed mission description comprising a list of operating points forthe gas turbine engines 10 during the mission. The mission descriptionmay be obtained from any suitable source, including an external sourcewith which the system 5000 is in communication, or a local memoryconfigured to store a range of different flight definitions.

The system 5000 further comprises an impact parameter obtaining module5004 configured to obtain nvPM impact parameters for the gas turbineengines. As discussed above, the impact parameters are associated witheach operating point of the proposed mission using compositions of fuelwhich include fuel from the first fuel source 302, fuel from the secondfuel source 304, or a blend thereof. The impact parameters may bedetermined as described above by accessing a lookup table. The lookuptable may be obtained from an external source by the system 5000 (e.g.so that it is tailored to a certain fuel available to the aircraft) oraccessed from a local memory.

The system 5000 further comprises a fuel characteristics calculatingmodule 5006. The fuel characteristics calculating module is configuredto calculate an optimised set of one or more fuel characteristics foreach operating point of the proposed mission defined in the missiondescription based on the nvPM impact parameters. This may be done byoptimising a cost function as described elsewhere herein.

The system 5000 further comprises a fuel allocation determining module5008. This is configured to receive the calculated fuel characteristicsfor each part of the mission and is configured to determine a fuelallocation based on them.

As discussed above, the first fuel characteristic may be associated witha level of nvPM production which is less than that of the second fuelcharacteristic. For example, the first fuel characteristic and thesecond fuel characteristic may be a percentage of SAF present in therespective fuel. Other fuel characteristics can be used as discussedabove in connection with the method of FIG. 34 .

Each of the operating points of the mission description obtained by themission description obtaining module 5002 may include any one or moreof: one or more operating conditions in which the gas turbine enginesare to operate, one or more fuel flow rate values corresponding to eachoperating point, and a time duration of operation at a correspondingoperating point.

The nvPM impact parameters obtained by the impact parameter obtainingmodule 5004 may each define an amount of nvPM produced by the gasturbine engines for different respective fuel characteristics comprisingthe first fuel, the second fuel, or a blend thereof at each operatingcondition of the mission description. The impact parameter obtainingmodule may obtain the impact parameters as described above, either froman external source, or stored in a local memory.

The fuel allocation that is determined by the fuel allocationdetermining module 5008 may be as described above, and may be determinedaccording to any of the factors discussed above in connection with theexample shown in FIG. 34 and so will not be repeated here. The system5000 may further comprise a fuel usage parameter determining module 5010configured to determine one or more fuel usage parameters as describedabove.

The fuel characteristics calculating module 5006 may be configured tocalculate the optimised set of one or more fuel characteristics byminimising a cost function dependent on the one or more nvPM impactparameters. The one or more nvPM impact parameters used by the fuelcharacteristics calculating module 5006 may be any of those describedherein.

Fleetwide Allocation of Fuel

In the examples above, the amount of fuel to be allocated to a specificmission is calculated to take advantage of fuel of differentcharacteristics. The inventors have further determined that availablefuel can be intelligently shared between a number of missions to furthermake advantageous use of different types of fuel available.

The present examples relate to determining a fuel allocation for aplurality of missions (i.e. the amount of fuel allocated to eachmission) carried out by a plurality of aircraft being supplied with fuelfrom a fuel source comprising an amount of a default fuel and an amountof a non-default fuel. The fuel source may comprise fuel storage vesselsor tanks from which aircraft are refuelled, the fuel storage vesselsholding default and non-default fuel separately e.g. the refuellingsource 60 shown in FIG. 4 and described above. The amount of each of thefuels is stored at a refuelling location at which aircraft used toperform the missions are refuelled. A finite supply of fuel is thereforeshared from that fuel source among the plurality of missions. This may,for example, be missions all leaving from the same airport, or from thesame terminal of an airport, which all have access to the same fuelsource. The fuel allocation may be determined for a plurality ofmissions over a predefined time window. The time window may representthe period over which the fuel available at the fuel source is to beused e.g. it may be the time period between deliveries of fuel to bestored in the fuel source and made available to the aircraft. In otherexamples, the time period may correspond to one or more banks ofoperations, or to a specific period such as one or more days, or one ormore weeks. The plurality of missions is referred to as a “fleet” whichmust be allocated fuel, and over which a fleet-wide optimisation is tobe performed.

The fuel available at the refuelling location may comprise a fixedamount of a default fuel and a fixed amount of a non-default fuel asdescribed above. The default fuel may be fossil kerosene (or other SAFpoor fuel), whereas the non-default fuel may be a SAF rich fuel e.g.having a SAF content of 50% or more, or 100% SAF (the SAF rich fuel maybe any fuel which has a greater proportion of SAF compared to the SAFpoor fuel). More generally, the non-default fuel may be associated witha level of nvPM production which is less than that of the default fuel(e.g. when being used in corresponding conditions). The default andnon-default fuels may have a variety of characteristics as discussedabove.

If both default fuel and non-default fuel is provided to a refuellinglocation, a number of possibilities exist for how those fuels could beused:

-   -   a) Mix all of the available non-default fuel in with a suitable        volume of default fuel in order to yield a single fuel        composition to be used by all aircraft being refuelled. This is        broadly what happens in the prior art at present.    -   b) Mix all of the available non-default fuel in with a suitable        volume of default fuel to satisfy the fuel requirement for        missions for which rich-burn engines are used. The missions with        lean-burn engines would receive or be allocated the default fuel        composition because for at least part of their operation the        lean-burn system reduces soot emissions very substantially.    -   c) As b), but in which the available non-default fuel is shared        across not only the rich-burn missions but also the subset of        lean-burn missions which use a “large” proportion of their fuel        in pilot-only mode (i.e. short-haul flights, or flights for        which the destination airport involves a high amount of        taxiing). “Large” may be defined with reference to a        predetermined threshold.    -   d) As c) but in which those lean-burn missions which receive an        allocation of non-default fuel use it solely in their pilot fuel        injectors and use the default fuel in their main fuel injectors.        For those missions receiving an allocation of non-default fuel,        this option corresponds to the example shown in FIG. 9 .

The inventors have determined that further advantages can be made of thefuel available at the refuelling location by determining an optimised“fleet-wide” fuel allocation for the plurality of missions which must beperformed using the fuel stored at the fuel source.

FIG. 37 illustrates a method 4090 of determining an optimised fleetwidefuel allocation for the plurality of missions introduced above. Themethod comprises obtaining 4092 an initial proposed fuel allocation foreach of the plurality of missions. The initial proposed fuel allocationmay define an amount of the non-default fuel and an amount of thedefault fuel allocated for each mission, and optionally a blending ratioat which the default and non-default fuel may be blended before loadingonto an aircraft. The proposed fuel allocation may have a correspondingfuel usage (e.g. a “mission fuel usage” made up of a fuel usageparameter for each flight condition of the mission as described above)which defines how the allocation is to be used over the duration of themission. In the case of aircraft with the appropriate capabilities, thismay include fuel compositions and proposed switching points and/orblending schedule to be employed on that mission. The fuel usageparameters may include a mixing ratio of the default and non-defaultfuel according to which fuel is to be mixed and loaded onto the aircraftto complete a respective mission.

The initial proposed fuel allocation for each mission may be determinedso that the available fuel is shared between the aircraft so each of themissions can be completed. The initial proposed fuel allocation mayprovide a starting point for further optimisation, and so does notnecessarily correspond to the optimum allocation for each of themissions. The initial fuel allocation may be done, for example,according to any of the methodologies a) to d) listed above.

Each of the plurality of missions is associated with a respectiveper-mission nvPM impact parameter. The per-mission impact parameter maybe determined based on the fuel allocation for the respective missionand the fuel usage defining how that fuel is to be used during therespective mission. The per-mission nvPM impact parameter may bedetermined by combining an nvPM impact parameter for each operatingpoint of the gas turbine engine(s) 10 of the respective aircraft 1during the mission (e.g. as defined in a mission description). Theper-mission nvPM impact parameters may be calculated as described belowor using the methods described above in connection with optimising fuelallocation for an individual mission.

Once an initial proposed fuel allocation for each of the missions hasbeen obtained, the method 4090 comprises performing 4094 a fleet-wideoptimisation in which the proposed fuel allocation of each of theplurality of missions is determined so that a combination of theper-mission nvPM impact is optimised within the constraints of the totalamount of default and non-default fuel to be allocated to the pluralityof missions. In some examples, the amount of default fuel may beconsidered unconstrained, with only the amount of the non-default fuelbeing limited.

The fuel allocation for each mission may be determined by modifying themfrom the initial proposed fuel allocation using an optimisation processso that the total (e.g. the sum) of the nvPM impact for all of theplurality of missions is minimised. This may be done using a suitabletype of optimisation process (e.g. an iterative optimisation) whichconverges on a set of per-mission fuel allocations that gives the lowestfleetwide nvPM impact. The fleet-wide optimisation results in anoptimised fuel usage being defined for each mission which makes the mosteffective use of the fuel available to minimise nvPM impact over all ofthe missions, rather than for each mission individually.

The method 4090 further comprises determining 4096 the fleetwide fuelallocation for the plurality of missions based on the fleet-wideoptimisation. The fleetwide fuel allocation may include fuel allocationfor each of the plurality of aircraft within the fleet. The allocationmay correspond to that defined above in the examples of determining fuelallocation for a single aircraft/mission. The fleetwide fuel allocationmay indicate the amount (either mass or volume) of the default fuel,non-default fuel or a mixture thereof that must be loaded onto theaircraft for each mission so that fuel to meet the optimised fuel usagerequirements for that mission is available. The fleetwide fuelallocation may also include the fuel usage for each aircraft to definehow the allocation of fuel loaded onto that aircraft is to be used.

As discussed above, the non-default fuel may be associated with a levelof nvPM production which is less than that of the default fuel. Morespecifically, the non-default fuel may be formed from a mixture of afirst fuel having a first fuel characteristic and a second fuel having adifferent fuel characteristic, different from the first. The first andsecond fuel characteristics may be a percentage of SAF within therespective fuel. The non-default fuel may therefore be a mixture of afirst fuel that is 100% SAF, and a second fuel which is 100% fossilkerosene. The non-default fuel may therefore be a SAF-rich fuel (forexample to give a SAF content in the non-default fuel of 50% or more),with the default fuel being a relatively SAF-poor fuel. In otherexamples, the non-default fuel may be a mixture of fuels that differ byany of the other fuel characteristics defined herein, for examplearomatic content.

Performing 4094 the fleet-wide optimisation may comprise performing amulti-parameter fleet-wide optimisation to minimise the fleetwide nvPMimpact i.e. the sum over all of the plurality of missions of per-missionnvPM impact within the constraints of the default and/or non-defaultfuel available for the plurality of missions. The optimisation maycomprise: i) performing 4097 a an outer-loop optimisation in which thefuel allocation of one or more of the missions is varied to reduce thesum of the per-mission nvPM impact parameters of the plurality ofmissions; and ii) performing 4097 b an inner-loop optimisation in whicha fuel usage for each of the missions is obtained according to theconstraints of the varied fuel allocation to determine a new proposedfuel usage for each of the plurality of missions. Obtaining the fuelusage may comprise determining an optimised fuel usage for the specificmission based on the varied fuel allocation in order to determine a newminimised per-mission nvPM impact parameter for that respective mission.This per-mission optimisation may be carried out as described below orusing any of the other techniques described herein. The fleet-wideoptimisation allows different distributions of SAF between the pluralityof missions to be tried (i.e. the outer loop optimisation), and then foreach proposed distribution each mission then explores how best to useits own proposed allocation (i.e. the inner loop optimisation).

The fleetwide optimisation may start with modifying the initial proposedfuel allocation obtained in step 4092. The fleetwide optimisation steps4097 a, 4097 b may be repeated until the method converges to anoptimised solution of the proposed fuel usage for each of the pluralityof missions which corresponds to the minimum value of the sum of theper-mission nvPM impact paraments. In each iteration of the outer-loopoptimisation 4097 a the fuel usage may be varied by varying a proposedquantity of non-default fuel allocated to the corresponding mission. Forexample, an amount of non-default fuel allocated to one mission may bemoved to another so that the non-default fuel can be allocated in a moreoptimal way overall.

In some examples, the inner loop optimisation 4097 b may compriseobtaining the fuel usage of a respective mission by obtaining the resultof a previous per-mission optimisation for that mission. This may allowthe computation time to be reduced. Performing the inner loopoptimisation may comprise obtaining a pre-prepared solution for the fuelusage for that mission (e.g. SAF percentages within the SAF-poor andSAF-rich fuel compositions for that mission, and the proposed switchingpoints and/or blending schedule to be employed on that mission) in orderto minimise the overall nvPM for that mission. The pre-prepared solutionmay be obtained from an approximate model, look-up table or “responsefunction” from which, for a given proposed fuel allocation, acorresponding pre-prepared solution can be obtained. In some examples,the pre-prepared solution may also be obtained based on other factorssuch as the proposed route for the respective mission and expectedweather conditions.

In some examples, the fleet-wide optimisation may be based at leastpartly on any one or more of:

-   -   i) a percentage of a first fuel having a first fuel        characteristic within the default fuel defining the lowest        possible percentage of fuel having the first fuel characteristic        which can be used for combustion;    -   ii) a percentage of the first fuel having the first fuel        characteristic within the non-default fuel defining the highest        possible percentage of fuel having the first fuel characteristic        which can be used for combustion; and/or    -   iii) the quantity of non-defuel available for the plurality of        missions.

Where the first fuel characteristic is the percentage SAF content of thefuel, factors i) and ii) above may allow the lowest and highest possibleSAF content of a fuel that can be formed from the default andnon-default fuel to be determined. For example, if the non-default fuelcontains 80% SAF and the default fuel contains 10% SAF, the greatestpossible SAF content of a mixture of the two fuels is 80%, and thelowest is 10%.

As discussed above, the fuel usage for each of the plurality of missionsis obtained by obtaining an optimised fuel usage for the respectivemission defining how the allocated fuel is used during the mission. Theoptimised fuel usage may include one or more fuel usage parameters basedon which the combustion system of the respective aircraft is controlled,or according to which fuel is mixed and loaded on to the aircraft. Forexample, the usage parameters may include SAF percentages within theSAF-poor and SAF-rich fuel compositions for that mission, and proposedswitching points and/or blending schedule to be employed on thatmission. The usage parameters may be chosen in order to minimise aper-mission nvPM impact parameter for that respective mission, withinthe constraints of the fuel allocated to that mission by the fuelallocation.

The optimised fuel usage for each mission (e.g. determined as part ofthe inner-loop optimisation) may be obtained by performing 4098 aper-mission optimisation as illustrated in FIG. 38 . The per-missionoptimisation may be performed by, for each respective mission:

-   -   i) determining 4098 a a type and/or operational capabilities of        a combustor used by the respective aircraft used for the        mission;    -   ii) determining 4098 b a total fuel requirement for the        respective mission;    -   iii) determining 4098 c an amount of fuel required for each type        of fuel injector provided in the combustor for the respective        mission;    -   iv) determining 4098 d the dependence of nvPM emissions for each        operating point of the engine using fuel having the        characteristics of the default fuel, non-default fuel, or a        mixture thereof; and    -   v) determining 4098 e an optimised fuel usage which minimises        the total nvPM emissions for the mission.

In step i), the type and capabilities of the combustor system theaircraft performing the respective mission are determined. This mayinclude determining whether the aircraft comprises a lean-burn stagedcombustor or a rich-burn combustor. It may further comprise determiningwhich combustor control modes are available. For example, it may bedetermined whether the operational capabilities include switching fuelbetween different sources being supplied to the pilot fuel injectors, orproviding a blended fuel to the pilot and/or main fuel injectors asdescribed in various examples herein. If the combustor is a lean-burnstaged combustor, determining an amount of fuel required for each typeof fuel injector comprises determining an amount of fuel required forthe pilot fuel injectors during pilot-only operation. It mayadditionally or alternatively comprise determining an amount of fuelrequired for the pilot fuel injectors during pilot-and-main operation.Determining an amount of fuel required for each type of combustor may,in some examples, comprise determining an amount of fuel required forthe pilot fuel injectors operating within a threshold range of theoperation at fuel flow rates below that of the staging point. This maybe the case in examples where a SAF rich fuel is provided to the pilotfuel injectors in pilot-only mode close to the staging point asdescribed above in connection with FIG. 15 . In step iv) the nvPMemission for each operating point of the engine dependent on thecharacteristics of the fuel being combustor is obtained. The engineoperating points may be as defined in a mission description as discussedelsewhere herein. In step v) the optimised fuel usage is determinedbased on the nvPM emission dependence in order to find the optimal fuelusage (e.g. type or blend of fuel used) at each engine operatingcondition to minimise the overall nvPM emissions. In some examples, thenvPM cost function for each engine operating condition as definedelsewhere herein may be minimised.

The information determined in each of the steps i) to iv) of theper-mission optimisation may be obtained or calculated using anysuitable technique that would be known to the skilled person. Forexample, this may include accessing information stored about therelevant aircraft and descriptions of each of the plurality of missionsthey are to fly.

The per-mission optimisation may be carried out according to any of theexamples given herein, for example that discussed in connection withFIG. 34 .

The method 4090 is a computer implemented method. The method 4090 may beimplemented by any suitable computing device, either on board anaircraft, separate from the aircraft as part of a fuel loading system,or as a dedicated system configured to manage fuel allocation.

The method 4090 may be part of a method 4100 of loading fuel onto theplurality of aircraft for which the fuel allocation has been determined.Such a method is illustrated in FIG. 39 . The method 4100 may comprisedetermining 4102 fuel allocation for the plurality of missions using themethod 4090 described above. Once the fuel allocation is determined, themethod 4100 comprises loading 4104 fuel onto the aircraft according tothe fuel allocation. This may comprise loading fuel by connecting theaircraft to a fuel supply using a known method. Loading of the fuel mayalso include providing fuel usage information calculated for eachmission to the aircraft for storage in an onboard control system (suchas the EEC 42) so that the aircraft can be controlled accordingly. Thismay include fuel usage information to allow the configuration of anyfuel tanks as required, selecting between fuels of differentcharacteristics at certain operating points, and/or a fuel blendingschedule. The usage information may also include a ratio at which thedefault and non-default fuel is to be mixed before it is loaded onto theaircraft (for example, in cases of aircraft for which onboard fuelmixing or selection is not possible). The step of loading the fuel mayalso comprise storing any required control parameters in a controlsystem of the relevant engine (e.g. the EEC 42).

FIG. 40 illustrates a fleetwide fuel allocation determination system5100 for determining a fuel allocation for a plurality of missions. Thesystem 5100 may carry out the method 4090 described above. Any featuredisclosed above in connection with the method 4090 may therefore alsoapply to the system 5100 of FIG. 40 .

The fleetwide fuel allocation determination system 5100 generallycomprises an initial proposed fuel allocation obtaining module 5102, afleetwide optimisation module 5104 and a fleetwide fuel allocationdetermining module 5106. These modules may carry out any of the stepsdefined above in respect to the method 4090 of determining a fleetwidefuel allocation.

The initial proposed fuel allocation obtaining module 5102 is configuredto obtain an initial proposed fuel allocation for each of the pluralityof missions. As discussed above, this may form the starting point forthe fleetwide optimisation. The fleetwide optimisation module 5104 isconfigured to perform a fleet-wide optimisation in which the proposedfuel allocation of each of the plurality of missions is modified withinthe constraints of the total available default and/or non-default fuelfrom the fuel source to minimise a sum of the per-mission nvPM impactparameters over all of the plurality of missions. Each of the pluralityof missions may be associated with a per-mission nvPM impact parameterdetermined according to the proposed fuel usage as described above.

The fleetwide fuel allocation determination module 5106 is configured todetermine the fleetwide fuel allocation for the plurality of missionsbased on the fleet-wide optimisation.

As discussed above, the non-default fuel is associated with a level ofnvPM production which is less than that of the default fuel. Thenon-default fuel is formed from a mixture of a first fuel having a firstfuel characteristic and a second fuel having a second fuelcharacteristic, different from the first. More specifically, the firstand second fuel characteristics may be a percentage of SAF within therespective fuel, and wherein the non-default fuel is a SAF-rich fuel andthe default fuel is a SAF-poor fuel.

The fleetwide optimisation module may be configured to perform thefleetwide optimisation as discussed above. The fleetwide optimisationmodule 5104 is therefore configured to perform the following steps:

-   -   i) perform an outer-loop optimisation in which the fuel        allocation of one or more of the missions is varied to reduce        the sum of the per-mission nvPM impact parameters of the        plurality of mission; and    -   ii) perform an inner-loop optimisation in which the fuel usage        for each of the missions is obtained according to the        constraints of the varied fuel allocation to determine a new        proposed fuel usage for each of the plurality of missions

The fleetwide optimisation module 5104 may be configured to repeat stepsi) and ii) until an optimised fuel usage for each of the plurality ofmissions is determined which corresponds to a minimised sum of theper-mission nvPM impact parameters. The fleetwide optimisation module5104 may be configured to perform the inner-loop optimisation byobtaining a pre-prepared solution for the fuel usage for a respectivemission. This may be obtained using information already available to thesystem 5100, such as that stored in a local or remote memory.

The fleetwide optimisation module 5104 may be configured to obtain thefuel usage for each of the plurality of missions by obtaining anoptimised fuel usage for the respective mission defining how the fuel isto be used for a respective mission in order to minimise a per-missionnvPM impact parameter for that mission.

The fleetwide optimisation module 5104 may be configured to obtain theoptimised fuel usage for each mission by performing a per-missionoptimisation as discussed above. In order to perform the per-missionoptimisation, the fleetwide optimisation module may perform the stepsshown in FIG. 38 , or as discussed in any of the examples above. Thedetails of the per-mission optimisation will not therefore be repeatedhere. Other optimisation techniques may be used, such as any of thosedescribed herein.

Any of the features described above in connection with the examples ofthe method 4090 of determining fleetwide fuel allocation may beperformed by the modules of the system 5100. Those features will nottherefore be described again here.

The computing modules, systems and computer implemented method stepsdescribed herein may be implemented in software executed by a processor,hardware or a combination of the two. In some embodiments, the modules,systems and method steps described herein may be implemented by one ormore computing devices. Such a computing device 6000 is illustrated inFIG. 41 , which includes one or more processors 6002, input/outputinterfaces 6004 and memory 6006. The memory may include computer ormachine readable memory forming a computer/machine readable medium. Theskilled person will appreciate that the memory may be provided by avariety of components including a volatile memory, a hard drive, anon-volatile memory, etc. The input/output interfaces may allowinformation such as a proposed mission description to be obtained froman external source, and may allow calculated fuel allocation or usageparameters to be output to anther device (e.g. to a fuel loading systemso that the fuel loading system can be controlled accordingly), or canbe output to a user.

The memory may store a set of computer readable instructions, datastructures, program modules or other data. The computer-readable mediamay not include temporary computer readable media (transitory media),such as a modulated data signal and a carrier wave.

The skilled person will understand that computing modules and systemsdescribed herein as individual components may not be physically separatefrom one another, and may be located at a single location or may bedistributed between several networked components. In some embodiments,the functionality of the modules/systems described herein may be divideddifferently between the modules/systems, or other modules/systemsprovided to perform any of the functions described herein.

In one aspect of the present application, there is provided amachine/computer readable medium or computer program product containinginstructions which, when read by a machine or computer, cause any of thecomputer implemented methods, or parts thereof, described or claimedherein to be performed.

The machine readable medium may be any of the following: a CDROM; a DVDROM/RAM (including -R/-RW or +R/+RW); a hard drive; a memory (includinga USB drive; an SD card; a compact flash card or the like); atransmitted signal (including an Internet download, ftp file transfer ofthe like); a wire; etc. The machine readable medium may be anon-transitory computer readable medium.

It will be understood that the invention is not limited to theembodiments above-described and various modifications and improvementscan be made without departing from the concepts described herein. Exceptwhere mutually exclusive, any of the features may be employed separatelyor in combination with any other features and the disclosure extends toand includes all combinations and sub-combinations of one or morefeatures described herein.

We claim:
 1. A computer implemented method of determining a fleetwidefuel allocation for a plurality of missions carried out by a pluralityof aircraft, the plurality of missions being supplied with fuel from afuel source comprising an amount of a default fuel and an amount of anon-default fuel, the fuel allocation indicating the amount of thenon-default fuel and the default fuel to be allocated to each of theplurality of missions, the default fuel and the non-default fuel havingone or more fuel characteristics different from each other, the methodcomprising: obtaining an initial proposed fuel allocation for each ofthe plurality of missions; performing a fleet-wide optimisation in whichthe proposed fuel allocation of each of the plurality of missions ismodified within the constraints of the total available default and/ornon-default fuel from the fuel source to minimise a sum of per-missionnvPM impact parameters over all of the plurality of missions, each ofthe plurality of missions being associated with a respective per-missionnvPM impact parameter determined according to a fuel usage for thatmission, the fuel usage defining how the fuel allocation for therespective mission is to be used during that mission; and determiningthe fleetwide fuel allocation for the plurality of missions based on thefleet-wide optimisation.
 2. The method according to claim 1, wherein thenon-default fuel is associated with a level of nvPM production which isless than that of the default fuel, wherein the non-default fuel isformed from a mixture of a first fuel having a first fuel characteristicand a second fuel having a second fuel characteristic, different fromthe first.
 3. The method according to claim 2, wherein the first andsecond fuel characteristics are a percentage of SAF within therespective fuel, and wherein the non-default fuel is a SAF-rich fuel andthe default fuel is a relatively SAF-poor fuel.
 4. The method accordingto claim 1, wherein performing the fleet-wide optimisation comprises: i)performing an outer-loop optimisation in which the fuel allocation ofone or more of the plurality of missions is varied to reduce the sum ofthe per-mission nvPM impact parameters of the plurality of missions; andii) performing an inner-loop optimisation in which the fuel usage foreach of the plurality of missions is obtained according to theconstraints of the varied fuel allocation to determine a new proposedfuel usage for each of the plurality of missions.
 5. The methodaccording to claim 4, wherein steps i) and ii) are repeated until anoptimised fuel usage for each of the plurality of missions is determinedwhich corresponds to a minimised sum of the per-mission nvPM impactparameters.
 6. The method according to claim 4, wherein the inner-loopoptimisation comprises obtaining a pre-prepared solution for the fuelusage for a respective mission.
 7. The method according to claim 4,wherein the proposed fuel allocation for each of the plurality ofmissions is obtained by obtaining an optimised fuel usage for therespective mission defining how the fuel is to be used to minimise theper-mission nvPM impact parameter for that mission.
 8. The methodaccording to claim 7, wherein the optimised fuel usage for each missionis obtained by performing a per-mission optimisation, the per-missionoptimisation optionally comprising, for each respective mission:determining a type and/or operational capabilities of a combustor usedby the respective aircraft used for the mission; determining a totalfuel requirement for the respective mission; determining an amount offuel required for each type of fuel injector provided in the combustorfor the respective mission where more than one type of injector isprovided; determining the dependence of nvPM emissions for each engineoperating point of the mission using fuel having the characteristics ofthe default fuel, non-default fuel, or a mixture thereof; anddetermining an optimised fuel usage which minimises the total nvPMemissions for the respective mission.
 9. The method according to claim8, wherein determining a type of combustor used by the aircraftcomprises determining whether the aircraft comprises a lean-burn stagedcombustor or a rich-burn combustor, and optionally wherein if thecombustor is a lean-burn staged combustor having pilot and main fuelinjectors, determining an amount of fuel required for each type of fuelinjector comprises: a) determining an amount of fuel required for thepilot injectors during pilot-and-main operation; and/or b) determiningan amount of fuel required for the pilot fuel injectors duringpilot-only operation; and/or c) determining an amount of fuel requiredfor the pilot fuel injectors operating within a threshold range of theoperation at fuel flow rates below that of the staging point.
 10. Themethod according to claim 1, wherein the fleet-wide optimisation isbased on: a percentage of a first fuel having a first fuelcharacteristic within the default fuel defining the lowest possiblepercentage of fuel having the first fuel characteristic which can beused for combustion; and/or a percentage of the first fuel having thefirst fuel characteristic within the non-default fuel defining thehighest possible percentage of fuel having the first fuel characteristicwhich can be used for combustion; and/or the quantity of non-defaultfuel available for the plurality of missions.
 11. A method of loadingfuel onto a plurality of aircraft carrying out a plurality of missions,the plurality of missions being supplied with fuel from a fuel sourcecomprising an amount of a default fuel and an amount of a non-defaultfuel, the method comprising: determining fuel allocation for theplurality of missions using the method of claim 1; and loading fuel ontothe plurality of aircraft according to the fuel allocation.
 12. Anon-transitory computer readable medium having stored thereoninstructions that, when executed by a processor, cause the processor toperform the method of claim
 1. 13. A fleetwide fuel allocationdetermination system for determining a fleet fuel allocation for aplurality of missions, the fleetwide fuel allocation determinationsystem comprising a computing device configured to perform the method ofclaim
 1. 14. A fleetwide fuel allocation determination system fordetermining a fuel allocation for a plurality of missions carried out bya plurality of aircraft, the plurality of missions being supplied withfuel from a fuel source comprising an amount of a default fuel and anamount of a non-default fuel, the fuel allocation indicating the amountof the non-default fuel and the default fuel to be allocated to each ofthe plurality of missions, the default fuel and the non-default fuelhaving one or more fuel characteristics different from each other, thesystem comprising: an initial proposed fuel allocation obtaining moduleconfigured to obtain an initial proposed fuel allocation for each of theplurality of missions; a fleetwide optimisation module configured toperform a fleet-wide optimisation in which the proposed fuel allocationof each of the plurality of missions is modified within the constraintsof the total available default and/or non-default fuel from the fuelsource to minimise a sum of per-mission nvPM impact parameters over allof the plurality of missions, each of the plurality of missions beingassociated with a respective per-mission nvPM impact parameterdetermined according to a proposed fuel usage for that mission, the fuelusage defining how the fuel allocation for the respective mission is tobe used during that mission; and a fleetwide fuel allocationdetermination module configured to determine the fleetwide fuelallocation for the plurality of missions based on the fleet-wideoptimisation.
 15. The system according to claim 14, wherein thenon-default fuel is associated with a level of nvPM production which isless than that of the default fuel, and optionally wherein thenon-default fuel is formed from a mixture of a first fuel having a firstfuel characteristic and a second fuel having a second fuelcharacteristic, different from the first, and further optionally whereinthe first and second fuel characteristics are a percentage of SAF withinthe respective fuel, and wherein the non-default fuel is a SAF-rich fueland the default fuel is a relatively SAF-poor fuel.
 16. The systemaccording to claim 14, wherein the fleetwide optimisation module isconfigured to perform the following steps: i) perform an outer-loopoptimisation in which the fuel allocation of one or more of theplurality of missions is varied to reduce the sum of the per-missionnvPM impact parameters of the plurality of missions; and ii) perform aninner-loop optimisation in which the fuel usage for each of theplurality missions is obtained according to the constraints of thevaried fuel allocation to determine a new proposed fuel usage for eachof the plurality of missions.
 17. The system according to claim 16,wherein one or both of: a) the fleetwide optimisation module isconfigured to repeat steps i) and ii) until an optimised fuel usage foreach of the plurality of missions is determined which corresponds to aminimised sum of the per-mission nvPM impact parameters; and/or b) thefleetwide optimisation module is configured to perform the inner-loopoptimisation by obtaining a pre-prepared solution for the fuel usage fora respective mission.
 18. The system according to claim 16, wherein thefleetwide optimisation module is configured to obtain the proposed fuelallocation for each of the plurality of missions by obtaining anoptimised fuel usage for the respective mission defining how the fuel isto be used in order to minimise the per-mission nvPM impact parameterfor that mission.
 19. The system according to claim 18, wherein thefleetwide optimisation module is configured to obtain the optimised fuelusage for each mission by performing a per-mission optimisation, theper-mission optimisation optionally comprising, for each respectivemission: determining a type and/or operational capabilities of acombustor used by the respective aircraft used for the mission;determining a total fuel requirement for the respective mission;determining an amount of fuel required for each type of fuel injectorprovided in the combustor for the respective mission where more than onetype of injector is provided; determining the dependence of nvPMemissions for each mission engine operating point of the mission usingfuel having the characteristics of the default fuel, non-default fuel,or a mixture thereof; and determining an optimised fuel usage whichminimises the total nvPM emissions for the respective mission.
 20. Thesystem according to claim 14, wherein the fleetwide optimisation moduleis configured to base the fleetwide optimisation on: a percentage of afirst fuel having a first fuel characteristic within the default fueldefining the lowest possible percentage of fuel having the first fuelcharacteristic which can be used for combustion; and/or a percentage ofthe first fuel having the first fuel characteristic within thenon-default fuel defining the highest possible percentage of fuel havingthe first fuel characteristic which can be used for combustion; and/orthe quantity of non-default fuel available for the plurality ofmissions.